Editor: Add no-js fallback for site editor.

Add a fallback condition with heading and error notice to handle a no JavaScript state for the site editor, comparable to what already exists in the post editor. 

Props afercia, joedolson, fencermonir, zebaafiashama, alexstine, rudlinkon.
Fixes #56228.
Built from https://develop.svn.wordpress.org/trunk@56025


git-svn-id: http://core.svn.wordpress.org/trunk@55537 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/site-editor.php

@@ -132,7 +132,30 @@ do_action( 'enqueue_block_editor_assets' );
 require_once ABSPATH . 'wp-admin/admin-header.php';
 ?>
 
-<div id="site-editor" class="edit-site"></div>
+<div class="edit-site">
+	<div id="site-editor"></div>
+
+	<?php // JavaScript is disabled. ?>
+	<div class="wrap hide-if-js site-editor-no-js">
+		<h1 class="wp-heading-inline"><?php _e( 'Edit site' ); ?></h1>
+		<div class="notice notice-error notice-alt">
+			<p>
+				<?php
+					/**
+					 * Filters the message displayed in the site editor interface when JavaScript is
+					 * not enabled in the browser.
+					 *
+					 * @since 6.3.0
+					 *
+					 * @param string  $message The message being displayed.
+					 * @param WP_Post $post    The post being edited.
+					 */
+					echo apply_filters( 'site_editor_no_javascript_message', __( 'The site editor requires JavaScript. Please enable JavaScript in your browser settings.' ), $post );
+				?>
+			</p>
+		</div>
+	</div>
+</div>
 
 <?php