Only add skip-link for block themes & templates on the frontend

[aristath]

Description

Fixes #32433

The problem is that we're checking with current_theme_supports( 'block-templates' ) which is by opt-out, and as a result, it's not a good way to detect a block theme or template.
To get around that, this PR adds a check to make sure that the $_wp_current_template_content global is set. That global var is only set when a block template gets rendered, so it's a way to detect if we're on a block template/theme or not.

How has this been tested?

• Tested a block theme - skip-link gets added ✔️
• Tested a classic theme - skip-link doesn't get added ✔️
• Tested a classic theme with a block template, content inside a group block using <main> - skip-link gets added ✔️

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[aristath]

Trac ticket - including patch for core: https://core.trac.wordpress.org/ticket/53368

[youknowriad]

how confident are you about the future-proof aspect of that check? Is there a way we can add a test for this?

[aristath]

Well... Ideally we'd have an is_block_template() function or something similar, but since we don't have something like that, checking for $_wp_current_template_content seems like the safest option.
That global var is only set in https://github.com/WordPress/WordPress/blob/4ae0e4220f0c01005afa0dcbb74979c0d5cb9abc/wp-includes/block-template.php#L45-L58 in WordPress Core, and

gutenberg/lib/full-site-editing/template-loader.php

Lines 61 to 74 in c746132

	if ( $block_template ) {
	if ( empty( $block_template->content ) && is_user_logged_in() ) {
	$_wp_current_template_content =
	sprintf(
	/* translators: %s: Template title */
	__( 'Empty template: %s', 'gutenberg' ),
	$block_template->title
	);
	} elseif ( ! empty( $block_template->content ) ) {
	$_wp_current_template_content = $block_template->content;
	}
	if ( isset( $_GET['_wp-find-template'] ) ) {
	wp_send_json_success( $block_template );
	}

in Gutenberg.
So in both cases it's only not-null when we're in a block template.

Even if we had a function is_block_template(), I can't think of any other way to do it except checking for that var 🤔

I'm pretty sure it's safe to use and future-proof - unless we change the implementation for block-templates completely

[youknowriad]

LGTM 👍

Maybe @mcsf could take a look as well.

[mcsf]

Thanks for the cc. This makes sense.