Adds filters to shortcircuit the GS public API

By doing this, we allow both the Gutenberg plugin and any 3rd party
to change the behavior of these functions. This is useful specially
for Gutenberg, where we may need to introduce future changes so
we need a way to hook into the core functions.

It's inspired by the pre_render_block filter.

lib/compat/wordpress-5.9/get-global-styles-and-settings.php

@@ -24,6 +24,11 @@
 	 * @return array The settings to retrieve.
 	 */
 	function wp_get_global_settings( $path = array(), $context = array() ) {
+		$pre_global_settings = apply_filters( 'pre_wp_get_global_settings', null, $path, $context );
+		if ( null !== $pre_global_settings ) {
+			return $pre_global_settings;
+		}
+
 		if ( ! empty( $context['block_name'] ) ) {
 			$path = array_merge( array( 'blocks', $context['block_name'] ), $path );
 		}
@@ -58,6 +63,11 @@ function wp_get_global_settings( $path = array(), $context = array() ) {
 	 * @return array The styles to retrieve.
 	 */
 	function wp_get_global_styles( $path = array(), $context = array() ) {
+		$pre_global_styles = apply_filters( 'pre_wp_get_global_styles', null, $path, $context );
+		if ( null !== $pre_global_styles ) {
+			return $pre_global_styles;
+		}
+
 		if ( ! empty( $context['block_name'] ) ) {
 			$path = array_merge( array( 'blocks', $context['block_name'] ), $path );
 		}
@@ -85,6 +95,11 @@ function wp_get_global_styles( $path = array(), $context = array() ) {
 	 * @return string Stylesheet.
 	 */
 	function wp_get_global_stylesheet( $types = array() ) {
+		$pre_global_stylesheet = apply_filters( 'pre_wp_get_global_stylesheet', null, $types );
+		if ( null !== $pre_global_stylesheet ) {
+			return $pre_global_stylesheet;
+		}
+
 		// Return cached value if it can be used and exists.
 		// It's cached by theme to make sure that theme switching clears the cache.
 		$can_use_cached = (