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.
Built from https://develop.svn.wordpress.org/trunk@50996
git-svn-id: http://core.svn.wordpress.org/trunk@50605 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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