Trac ticket: https://core.trac.wordpress.org/ticket/64389

When a theme has a theme.json, then it will not emit the classic-theme-styles stylesheet. When no other block styles printed either, as in the case of Elementor with the Hello Elementor theme, then the existing logic in wp_hoist_late_printed_styles() does not have a reference point (from printed block styles) to know where it should inject the global-styles. So, this PR makes the hoisting global-styles much more robust by enqueueing a placeholder inline style which then gets replaced during processing of the template enhancement output buffer. This also allows for removing some convoluted if statements to conditionally inject global styles around other styles.

The same goes for where non-core block styles get printed. Previously the logic here would attempt to locate the classic-theme-styles and then insert the block styles after that style. But if the style was not enqueued, then it would have fall back to trying to insert them after elsewhere. This, again, is simplified by printing a placeholder inline style at the point where block styles normally get inserted. This placeholder style in the HEAD is replaced with the styles rendered in the footer.

The placeholder inline styles are simply removed if there are no appropriate styles to hoist there.

How I tested:

0. Deactivate Gutenberg, because it does not have a patched gutenberg_enqueue_global_styles() to correspond with the modified wp_enqueue_global_styles() in this PR. Or else, have Sync changes from wp_enqueue_global_styles() to Gutenberg override gutenberg#76127 checked out.
1. Install and activate the Elementor plugin.
2. Install and activate the Hello Elementor theme.
3. Override the Elementor hotfix logic by adding plugin that does: add_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true' );
4. It seems a change in trunk eliminated the underline from links (probably Remove link underline style from default theme.json gutenberg#74901), so add this plugin code to restore the previous underline to reproduce the bug with the cascade:

add_filter(
	'print_styles_array',
	static function ( $handles ) {
		if ( in_array( 'global-styles', $handles ) ) {
			wp_add_inline_style( 'global-styles', 'a { text-decoration: underline; }' );
		}
		return $handles;
	}
);

5. Create a blank post with Elementor.
6. View the post on the frontend.

❌ Before (links are underlined)	✅ After (links are not underlined)
STYLE#global-styles-inline-css is printed at the end of HEAD, far after LINK#hello-elementor-css	STYLE#global-styles-inline-css element is printed after STYLE#wp-emoji-styles-inline-css and before LINK#hello-elementor-css.

Fixing Cascade for wp-block-library added inline styles

This PR also fixes (via 48eb4f6) the cascade for when inline styles are added to wp-block-library. When the combined block library is enqueued, theme authors would expect any inline style to appear in the cascade after any core block styles. However, in 6.9 this was no longer the case as the separate block styles would get inserted after any such wp-block-library inline styles. This is fixed now so that the inline styles for wp-block-library get after any separate core block styles.

For example, consider this plugin code:

add_action(
	'enqueue_block_assets',
	static function () {
		// Extra CSS which prevents empty inline style containing placeholder from being removed.
		wp_add_inline_style( 'wp-block-library', '.wp-block-separator{ outline:solid 1px lime; }' );
	}
);

Before 6.9, in a classic theme where combined block library is loaded, this would result in the following styles printed (e.g. in Twenty Twenty-One):

<link rel='stylesheet' id='wp-block-library-css' href='http://localhost:8000/wp-includes/css/dist/block-library/style.css?ver=7.0-beta2-61752-src' media='all' />
<style id="wp-block-library-inline-css">
.wp-block-separator{ outline:solid 1px lime; }
/*# sourceURL=wp-block-library-inline-css */
</style>
<link rel='stylesheet' id='wp-block-library-theme-css' href='http://localhost:8000/wp-includes/css/dist/block-library/theme.css?ver=7.0-beta2-61752-src' media='all' />
<link rel='stylesheet' id='classic-theme-styles-css' href='http://localhost:8000/wp-includes/css/classic-themes.css?ver=7.0-beta2-61752-src' media='all' />

In 6.9, however, when separate block styles are loaded on demand, the lime color style appears before the block style for the Separator block:

<link rel='stylesheet' id='wp-block-library-css' href='http://localhost:8000/wp-includes/css/dist/block-library/common.css?ver=7.0-beta2-61752-src' media='all' />
<style id="wp-block-library-inline-css">
.wp-block-separator{ outline:solid 1px lime; }

/*# sourceURL=wp-block-library-inline-css */
</style>
<link rel='stylesheet' id='wp-block-separator-css' href='http://localhost:8000/wp-includes/blocks/separator/style.css?ver=7.0-beta2-61752-src' media='all' />
<link rel='stylesheet' id='wp-block-separator-theme-css' href='http://localhost:8000/wp-includes/blocks/separator/theme.css?ver=7.0-beta2-61752-src' media='all' />

<link rel='stylesheet' id='classic-theme-styles-css' href='http://localhost:8000/wp-includes/css/classic-themes.css?ver=7.0-beta2-61752-src' media='all' />

With the changes in this PR, the correct order placement in the cascade is better preserved:

<link rel='stylesheet' id='wp-block-library-css' href='http://localhost:8000/wp-includes/css/dist/block-library/common.css?ver=7.0-beta2-61752-src' media='all' />

<link rel='stylesheet' id='wp-block-separator-css' href='http://localhost:8000/wp-includes/blocks/separator/style.css?ver=7.0-beta2-61752-src' media='all' />
<link rel='stylesheet' id='wp-block-separator-theme-css' href='http://localhost:8000/wp-includes/blocks/separator/theme.css?ver=7.0-beta2-61752-src' media='all' />
<style id="wp-block-library-inline-css-extra">
.wp-block-separator{ outline:solid 1px lime; }
/*# sourceURL=wp-block-library-inline-css */
</style>

<link rel='stylesheet' id='classic-theme-styles-css' href='http://localhost:8000/wp-includes/css/classic-themes.css?ver=7.0-beta2-61752-src' media='all' />

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.