Widgets: Add missing no JS notice in Widgets screen.

Add an H1 heading and an admin notice to display on the Widgets screen when JS is not available. Invite the user to either install or activate the Classic Widgets plugin, as that plugin provides basic functionality without JS.

Props afercia, nihar007, huzaifaalmesbah, joedolson, matthewfarlymn, bvreeman22, bosskhj, devmuhib, shailu25, joedolson.
Fixes #58738.

git-svn-id: https://develop.svn.wordpress.org/trunk@56671 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/widgets-form-blocks.php

@@ -73,7 +73,50 @@
 do_action( 'widgets_admin_page' );
 ?>
 
-<div id="widgets-editor" class="blocks-widgets-container"></div>
+<div id="widgets-editor" class="blocks-widgets-container">
+	<?php // JavaScript is disabled. ?>
+	<div class="wrap hide-if-js widgets-editor-no-js">
+		<h1 class="wp-heading-inline"><?php echo esc_html( $title ); ?></h1>
+		<?php
+		if ( file_exists( WP_PLUGIN_DIR . '/classic-widgets/classic-widgets.php' ) ) {
+			// If Classic Widgets is already installed, provide a link to activate the plugin.
+			$installed           = true;
+			$plugin_activate_url = wp_nonce_url( 'plugins.php?action=activate&amp;plugin=classic-widgets/classic-widgets.php', 'activate-plugin_classic-widgets/classic-widgets.php' );
+			$message             = sprintf(
+				/* translators: %s: Link to activate the Classic Widgets plugin. */
+				__( 'The block widgets require JavaScript. Please enable JavaScript in your browser settings, or activate the <a href="%s">Classic Widgets plugin</a>.' ),
+				esc_url( $plugin_activate_url )
+			);
+		} else {
+			// If Classic Widgets is not installed, provide a link to install it.
+			$installed          = false;
+			$plugin_install_url = wp_nonce_url( self_admin_url( 'update.php?action=install-plugin&plugin=classic-widgets' ), 'install-plugin_classic-widgets' );
+			$message            = sprintf(
+				/* translators: %s: A link to install the Classic Widgets plugin. */
+				__( 'The block widgets require JavaScript. Please enable JavaScript in your browser settings, or install the <a href="%s">Classic Widgets plugin</a>.' ),
+				esc_url( $plugin_install_url )
+			);
+		}
+		/**
+		 * Filters the message displayed in the block widget interface when JavaScript is
+		 * not enabled in the browser.
+		 *
+		 * @since 6.4.0
+		 *
+		 * @param string $message The message being displayed.
+		 * @param bool   $installed Whether the Classic Widget plugin is installed.
+		 */
+		$message = apply_filters( 'block_widgets_no_javascript_message', $message, $installed );
+		wp_admin_notice(
+			$message,
+			array(
+				'type'               => 'error',
+				'additional_classes' => array( 'hide-if-js' ),
+			)
+		);
+		?>
+	</div>
+</div>
 
 <?php
 /** This action is documented in wp-admin/widgets-form.php */