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/script-loader.php

@@ -1552,6 +1552,20 @@ function wp_default_styles( $styles ) {
 		'list-reusable-blocks' => array( 'wp-components' ),
 		'reusable-blocks'      => array( 'wp-components' ),
 		'nux'                  => array( 'wp-components' ),
+		'edit-widgets'         => array(
+			'wp-components',
+			'wp-block-editor',
+			'wp-edit-blocks',
+			'wp-block-library',
+			'wp-reusable-blocks',
+		),
+		'customize-widgets'    => array(
+			'wp-components',
+			'wp-block-editor',
+			'wp-edit-blocks',
+			'wp-block-library',
+			'wp-reusable-blocks',
+		),
 	);
 
 	foreach ( $package_styles as $package => $dependencies ) {