Add: Global Styles Rest endpoints.

This commit ports the global styles rest endpoints from the Gutenberg plugin into the core.

See #54336.
Props oandregal, aristath, timothyblynjacobs, spacedmonkey, youknowriad.

git-svn-id: https://develop.svn.wordpress.org/trunk@52051 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -301,6 +301,10 @@ function create_initial_rest_routes() {
 	$controller = new WP_REST_Block_Types_Controller();
 	$controller->register_routes();
 
+	// Global Styles.
+	$controller = new WP_REST_Global_Styles_Controller;
+	$controller->register_routes();
+
 	// Settings.
 	$controller = new WP_REST_Settings_Controller;
 	$controller->register_routes();