Fix prioritization of TT1 stylesheet after core theme sanitizer changes

[westonruter]

Summary

While working on #6624, I realized that in Twenty Twenty-One the theme stylesheet was getting excluded when a large plugin stylesheet was present earlier in the document. This ended up being a bug with the prioritization logic caused by the core theme sanitizer, where we set the src of the stylesheet to false to make an alias that isn't printed but which we then attach inline styles for the entire stylesheet source with fixes for AMP compatibility.

So this adds a special case for what the plugin uniquely does with the Twenty Twenty-One.

Considering the following plugin being active:

<?php
/**
 * Plugin Name: Add Style To Beginning of Head
 */

add_action('wp_head', function () {
	?>
	<style>
		body:after {
			content: '<?php echo str_repeat( 'a', 28324 ) ?>';
		}
	</style>
	<?php
}, 0);

Before	After

Notice the Twenty Twenty-One style now has the proper priority and thus it is not selected for exclusion.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

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[delawski]

QA Passed

With TT1 theme active, after activating a script like this:

<?php
/**
 * Plugin Name: Add Style To Beginning of Head
 */

add_action('wp_head', function () {
	?>
	<style>
		body:after {
			content: '<?php echo str_repeat( 'a', 68324 ) ?>';
		}
	</style>
	<?php
}, 0);

and revalidating one of the URLs, the excessive CSS issue was attributed to the plugin and not to the theme:

✅ The TT1 stylesheet has not been excluded:

Tested on AMP Version 2.2.0-alpha-20211123T020732Z-5405daa