Webfonts: redesign architecture to use WP_Dependencies (#43492)

Complete redesign of the Webfonts API:

* The current implementation re-implements a lot of the concepts that are already handled by `WP_Dependencies` for the script and style loaders. The script loader classes already have the concepts of dependencies, enqueuing, and registering, as well as a way to "do_item()" (generate CSS or a tag with source, in this case).

* Includes a temporary backwards-compatible (BC) layer to give extenders time and information to upgrade their code to the new API data structures and functionality.

Props: hellofromtonya, aristath, desrosj, colorful-tones, ironprogrammer.

lib/compat/wordpress-6.2/script-loader.php

@@ -127,6 +127,30 @@ function gutenberg_resolve_assets_override() {
 
 	$scripts = ob_get_clean();
 
+	/*
+	 * Generate web font @font-face styles for the site editor iframe.
+	 * Use the registered font families for printing.
+	 */
+	if ( class_exists( 'WP_Web_Fonts' ) ) {
+		$wp_webfonts = wp_webfonts();
+		$registered  = $wp_webfonts->get_registered_font_families();
+		if ( ! empty( $registered ) ) {
+			$queue = $wp_webfonts->queue;
+			$done  = $wp_webfonts->done;
+
+			$wp_webfonts->done  = array();
+			$wp_webfonts->queue = $registered;
+
+			ob_start();
+			$wp_webfonts->do_items();
+			$styles .= ob_get_clean();
+
+			// Reset the Web Fonts API.
+			$wp_webfonts->done  = $done;
+			$wp_webfonts->queue = $queue;
+		}
+	}
+
 	return array(
 		'styles'  => $styles,
 		'scripts' => $scripts,