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.


git-svn-id: https://develop.svn.wordpress.org/trunk@50995 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets/class-wp-widget-custom-html.php

@@ -45,6 +45,7 @@ class WP_Widget_Custom_HTML extends WP_Widget {
 			'classname'                   => 'widget_custom_html',
 			'description'                 => __( 'Arbitrary HTML code.' ),
 			'customize_selective_refresh' => true,
+			'show_instance_in_rest'       => true,
 		);
 		$control_ops = array(
 			'width'  => 400,