HTML API: Scan all syntax tokens in a document, read modifiable text.

Since its introduction in WordPress 6.2 the HTML Tag Processor has
provided a way to scan through all of the HTML tags in a document and
then read and modify their attributes. In order to reliably do this, it
also needed to be aware of other kinds of HTML syntax, but it didn't
expose those syntax tokens to consumers of the API.

In this patch the Tag Processor introduces a new scanning method and a
few helper methods to read information about or from each token. Most
significantly, this introduces the ability to read `#text` nodes in the
document.

What's new in the Tag Processor?
================================

 - `next_token()` visits every distinct syntax token in a document.
 - `get_token_type()` indicates what kind of token it is.
 - `get_token_name()` returns something akin to `DOMNode.nodeName`.
 - `get_modifiable_text()` returns the text associated with a token.
 - `get_comment_type()` indicates why a token represents an HTML comment.

Example usage.
==============

```php
function strip_all_tags( $html ) {
        $text_content = '';
        $processor    = new WP_HTML_Tag_Processor( $html );

        while ( $processor->next_token() ) {
                if ( '#text' !== $processor->get_token_type() ) {
                        continue;
                }

                $text_content .= $processor->get_modifiable_text();
        }

        return $text_content;
}
```

What changes in the Tag Processor?
==================================

Previously, the Tag Processor would scan the opening and closing tag of
every HTML element separately. Now, however, there are special tags
which it only visits once, as if those elements were void tags without
a closer.

These are special tags because their content contains no other HTML or
markup, only non-HTML content.

 - SCRIPT elements contain raw text which is isolated from the rest of
   the HTML document and fed separately into a JavaScript engine. There
   are complicated rules to avoid escaping the script context in the HTML.
   The contents are left verbatim, and character references are not decoded.

 - TEXTARA and TITLE elements contain plain text which is decoded
   before display, e.g. transforming `&` into `&`. Any markup which
   resembles tags is treated as verbatim text and not a tag.

 - IFRAME, NOEMBED, NOFRAMES, STYLE, and XMP elements are similar to the
   textarea and title elements, but no character references are decoded.
   For example, `&` inside a STYLE element is passed to the CSS engine
   as the literal string `&` and _not_ as `&`.

Because it's important not treat this inner content separately from the
elements containing it, the Tag Processor combines them when scanning
into a single match and makes their content available as modifiable
text (see below).

This means that the Tag Processor will no longer visit a closing tag for
any of these elements unless that tag is unexpected.

    <title>There is only a single token in this line</title>
    <title>There are two tokens in this line></title></title>
    </title><title>There are still two tokens in this line></title>

What are tokens?
================

The term "token" here is a parsing term, which means a primitive unit in
HTML. There are only a few kinds of tokens in HTML:

 - a tag has a name, attributes, and a closing or self-closing flag.
 - a text node, or `#text` node contains plain text which is displayed
   in a browser and which is decoded before display.
 - a DOCTYPE declaration indicates how to parse the document.
 - a comment is hidden from the display on a page but present in the HTML.

There are a few more kinds of tokens that the HTML Tag Processor will
recognize, some of which don't exist as concepts in HTML. These mostly
comprise XML syntax elements that aren't part of HTML (such as CDATA and
processing instructions) and invalid HTML syntax that transforms into
comments.

What is a funky comment?
========================

This patch treats a specific kind of invalid comment in a special way.
A closing tag with an invalid name is considered a "funky comment." In
the browser these become HTML comments just like any other, but their
syntax is convenient for representing a variety of bits of information
in a well-defined way and which cannot be nested or recursive, given
the parsing rules handling this invalid syntax.

 - `</1>`
 - `</%avatar_url>`
 - `</{"wp_bit": {"type": "post-author"}}>`
 - `</[post-author]>`
 - `</__( 'Save Post' );>`

All of these examples become HTML comments in the browser. The content
inside the funky content is easily parsable, whereby the only rule is
that it starts at the `<` and continues until the nearest `>`. There
can be no funky comment inside another, because that would imply having
a `>` inside of one, which would actually terminate the first one.

What is modifiable text?
========================

Modifiable text is similar to the `innerText` property of a DOM node.
It represents the span of text for a given token which may be modified
without changing the structure of the HTML document or the token.

There is currently no mechanism to change the modifiable text, but this
is planned to arrive in a later patch.

Tags
====

Most tags have no modifiable text because they have child nodes where
text nodes are found. Only the special tags mentioned above have
modifiable text.

    <div class="post">Another day in HTML</div>
    └─ tag ──────────┘└─ text node ─────┘└────┴─ tag

    <title>Is <img> &gt; <image>?</title>
    │      └ modifiable text ───┘       │ "Is <img> > <image>?"
    └─ tag ─────────────────────────────┘

Text nodes
==========

Text nodes are entirely modifiable text.

    This HTML document has no tags.
    └─ modifiable text ───────────┘

Comments
========

The modifiable text inside a comment is the portion of the comment that
doesn't form its syntax. This applies for a number of invalid comments.

    <!-- this is inside a comment -->
    │   └─ modifiable text ──────┘  │
    └─ comment token ───────────────┘

    <!-->
    This invalid comment has no modifiable text.

    <? this is an invalid comment -->
    │ └─ modifiable text ────────┘  │
    └─ comment token ───────────────┘

    <[CDATA[this is an invalid comment]]>
    │       └─ modifiable text ───────┘ │
    └─ comment token ───────────────────┘

Other token types also have modifiable text. Consult the code or tests
for further information.

src/wp-includes/html-api/class-wp-html-processor.php

@@ -149,17 +149,6 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 */
 	const MAX_BOOKMARKS = 100;
 
-	/**
-	 * Static query for instructing the Tag Processor to visit every token.
-	 *
-	 * @access private
-	 *
-	 * @since 6.4.0
-	 *
-	 * @var array
-	 */
-	const VISIT_EVERYTHING = array( 'tag_closers' => 'visit' );
-
 	/**
 	 * Holds the working state of the parser, including the stack of
 	 * open elements and the stack of active formatting elements.
@@ -424,6 +413,30 @@ public function next_tag( $query = null ) {
 		return false;
 	}
 
+	/**
+	 * Ensures internal accounting is maintained for HTML semantic rules while
+	 * the underlying Tag Processor class is seeking to a bookmark.
+	 *
+	 * This doesn't currently have a way to represent non-tags and doesn't process
+	 * semantic rules for text nodes. For access to the raw tokens consider using
+	 * WP_HTML_Tag_Processor instead.
+	 *
+	 * @since 6.5.0 Added for internal support; do not use.
+	 *
+	 * @access private
+	 *
+	 * @return bool
+	 */
+	public function next_token() {
+		$found_a_token = parent::next_token();
+
+		if ( '#tag' === $this->get_token_type() ) {
+			$this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		return $found_a_token;
+	}
+
 	/**
 	 * Indicates if the currently-matched tag matches the given breadcrumbs.
 	 *
@@ -520,7 +533,9 @@ public function step( $node_to_process = self::PROCESS_NEXT_NODE ) {
 				$this->state->stack_of_open_elements->pop();
 			}
 
-			parent::next_tag( self::VISIT_EVERYTHING );
+			while ( parent::next_token() && '#tag' !== $this->get_token_type() ) {
+				continue;
+			}
 		}
 
 		// Finish stepping when there are no more tokens in the document.