REST API: Add widget endpoints

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

Fixes #41683.
Props TimothyBlynJacobs, spacedmonkey, zieladam, jorgefilipecosta, youknowriad, kevin940726.

Built from https://develop.svn.wordpress.org/trunk@50995


git-svn-id: http://core.svn.wordpress.org/trunk@50604 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/widgets/class-wp-widget-recent-posts.php

@@ -26,6 +26,7 @@ class WP_Widget_Recent_Posts extends WP_Widget {
 			'classname'                   => 'widget_recent_entries',
 			'description'                 => __( 'Your site’s most recent Posts.' ),
 			'customize_selective_refresh' => true,
+			'show_instance_in_rest'       => true,
 		);
 		parent::__construct( 'recent-posts', __( 'Recent Posts' ), $widget_ops );
 		$this->alt_option_name = 'widget_recent_entries';