Widgets: Add support for the Widgets Editor on after_setup_theme instead of widgets_init.

This better aligns with developer expectations, as `add_theme_support()` and `remove_theme_support()` are meant to be called within functions attached to the `after_setup_theme` hook.

This also adds the `widgets-block-editor` feature to the docblock for `add_theme_support()`. 

Props kevin940726, caseymilne, jamesros161, noisysocks, Mamaduka, audrasjb, zieladam, hellofromTonya, desrosj.
Fixes #53424.

git-svn-id: https://develop.svn.wordpress.org/trunk@51214 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets.php

@@ -1801,8 +1801,6 @@ function wp_widgets_init() {
 
 	register_widget( 'WP_Widget_Block' );
 
-	add_theme_support( 'widgets-block-editor' );
-
 	/**
 	 * Fires after all default WordPress widgets have been registered.
 	 *
@@ -1811,6 +1809,18 @@ function wp_widgets_init() {
 	do_action( 'widgets_init' );
 }
 
+/**
+ * Enables the widgets block editor. This is hooked into 'after_setup_theme' so
+ * that the block editor is enabled by default but can be disabled by themes.
+ *
+ * @since 5.8.0
+ *
+ * @access private
+ */
+function wp_setup_widgets_block_editor() {
+	add_theme_support( 'widgets-block-editor' );
+}
+
 /**
  * Whether or not to use the block editor to manage widgets. Defaults to true
  * unless a theme has removed support for widgets-block-editor or a plugin has