Adds the widgets block editor to widgets.php and customize.php

Moves the widgets block editor from Gutenberg into WordPress Core.

- Adds @wordpress/edit-widgets, @wordpress/customize-widgets and
  @wordpress/widgets.
- Modifies wp-admin/widgets.php to branch between the old editor and new editor
  depending on wp_use_widgets_block_editor().
- Modifies WP_Customize_Widgets to branch between the old editor control and new
  editor control depending on wp_use_widgets_block_editor().

Follows [50996] which was missing these files.
See #51506.
Props isabel_brison, TimothyBlynJacobs, andraganescu, kevin940726, talldanwp.

Built from https://develop.svn.wordpress.org/trunk@50997


git-svn-id: http://core.svn.wordpress.org/trunk@50606 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/customize/class-wp-sidebar-block-editor-control.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Customize API: WP_Sidebar_Block_Editor_Control class.
+ *
+ * @package WordPress
+ * @subpackage Customize
+ * @since 5.8.0
+ */
+
+/**
+ * Core class used to implement the widgets block editor control in the
+ * customizer.
+ *
+ * @since 5.8.0
+ *
+ * @see WP_Customize_Control
+ */
+class WP_Sidebar_Block_Editor_Control extends WP_Customize_Control {
+	/**
+	 * The control type.
+	 *
+	 * @since 5.8.0
+	 *
+	 * @var string
+	 */
+	public $type = 'sidebar_block_editor';
+
+	/**
+	 * Render the widgets block editor container.
+	 *
+	 * @since 5.8.0
+	 */
+	public function render_content() {
+		// Render an empty control. The JavaScript in
+		// @wordpress/customize-widgets will do the rest.
+	}
+}