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().

Fixes #51506.
Props isabel_brison, TimothyBlynJacobs, andraganescu, kevin940726, talldanwp.


git-svn-id: https://develop.svn.wordpress.org/trunk@50996 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-editor.php

@@ -225,6 +225,40 @@ function get_default_block_editor_settings() {
 		$editor_settings['gradients'] = $gradient_presets;
 	}
 
+	/**
+	 * Filters the list of widget-type IDs that should **not** be offered by the
+	 * Legacy Widget block.
+	 *
+	 * Returning an empty array will make all widgets available.
+	 *
+	 * @since 5.8.0
+	 *
+	 * @param array $widgets An array of excluded widget-type IDs.
+	 */
+	$editor_settings['widgetTypesToHideFromLegacyWidgetBlock'] = apply_filters(
+		'widget_types_to_hide_from_legacy_widget_block',
+		array(
+			'pages',
+			'calendar',
+			'archives',
+			'media_audio',
+			'media_image',
+			'media_gallery',
+			'media_video',
+			'meta',
+			'search',
+			'text',
+			'categories',
+			'recent-posts',
+			'recent-comments',
+			'rss',
+			'tag_cloud',
+			'nav_menu',
+			'custom_html',
+			'block',
+		)
+	);
+
 	return $editor_settings;
 }