REST API: Add widget endpoints

Adds the sidebars, widgets and widget-types REST API endpoints from the
Gutenberg plugin.

Fixes #41683.
Props TimothyBlynJacobs.

Built from https://develop.svn.wordpress.org/trunk@50993


git-svn-id: http://core.svn.wordpress.org/trunk@50602 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api.php

@@ -313,6 +313,18 @@ function create_initial_rest_routes() {
 	$controller = new WP_REST_Plugins_Controller();
 	$controller->register_routes();
 
+	// Sidebars.
+	$controller = new WP_REST_Sidebars_Controller();
+	$controller->register_routes();
+
+	// Widget Types.
+	$controller = new WP_REST_Widget_Types_Controller();
+	$controller->register_routes();
+
+	// Widgets.
+	$controller = new WP_REST_Widgets_Controller();
+	$controller->register_routes();
+
 	// Block Directory.
 	$controller = new WP_REST_Block_Directory_Controller();
 	$controller->register_routes();