Editor: Add support for custom CSS in global styles.

This changeset introduces functions `wp_get_global_styles_custom_css()` and `wp_enqueue_global_styles_custom_css()`, which allow accessing and enqueuing custom CSS added via global styles.

Custom CSS via global styles is handled separately from custom CSS via the Customizer. If a site uses both features, the custom CSS from both sources will be loaded. The global styles custom CSS is then loaded after the Customizer custom CSS, so if there are any conflicts between the rules, the global styles take precedence.

Similarly to e.g. [55185], the result is cached in a non-persistent cache, except when `WP_DEBUG` is on to avoid interrupting the theme developer's workflow.

Props glendaviesnz, oandregal, ntsekouras, mamaduka, davidbaumwald, hellofromtonya, flixos90.
Fixes #57536.


git-svn-id: https://develop.svn.wordpress.org/trunk@55192 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-editor.php

@@ -410,6 +410,16 @@ function get_block_editor_settings( array $custom_settings, $block_editor_contex
 			$block_classes['css'] = $actual_css;
 			$global_styles[]      = $block_classes;
 		}
+
+		/*
+		 * Add the custom CSS as a separate stylesheet so any invalid CSS
+		 * entered by users does not break other global styles.
+		 */
+		$editor_settings['styles'][] = array(
+			'css'            => wp_get_global_styles_custom_css(),
+			'__unstableType' => 'user',
+			'isGlobalStyles' => true,
+		);
 	} else {
 		// If there is no `theme.json` file, ensure base layout styles are still available.
 		$block_classes = array(