feat(html): Add escape and unescape functions for HTML entities

[lionel-rowe]

Fixes #3329

[lino-levan]

Maybe we should rename html_entities to just entities since it is in the html top level folder?

[iuioiua]

Maybe we should rename html_entities to just entities since it is in the html top level folder?

+1

[lionel-rowe]

Bikeshedding: encode/decode vs. escape/unescape vs [any other options]? I don't have a strong preference.

[timreichen]

+1 for escape and unescape

[kt3k]

LGTM

[kt3k]

Memo:

• php - htmlspecialchars, htmlspecialchars_decode (global builtins)
• ruby - CGI.escapeHTML, CGI.unescapeHTML
• golang - EscapeString, UnescapeString (html standard library)
• python - html.escape, html.unescape (html standard library)

[lionel-rowe]

Memo:

• php - htmlspecialchars, htmlspecialchars_decode (global builtins)
• ruby - CGI.escapeHTML, CGI.unescapeHTML
• golang - EscapeString, UnescapeString (html standard library)
• python - html.escape, html.unescape (html standard library)

Honorable mention also to htmlentities and html_entity_decode in PHP, which use a full list of named entities.

Also worth considering the bit of the HTML spec that deals with escaping text nodes and attribute nodes:

1. Replace any occurrence of the & character by the string &.
2. Replace any occurrences of the U+00A0 NO-BREAK SPACE character by the string  .
3. If the algorithm was invoked in the attribute mode, replace any occurrences of the " character by the string ".
4. If the algorithm was not invoked in the attribute mode, replace any occurrences of the < character by the string <, and any occurrences of the > character by the string >.

[kt3k]

@lionel-rowe Thanks for your contribution!