Resolve #3416770 "Add lbblockdecorator support"

Closes #3416770

modules/lb_plus_lb_block_decorator/src/EventSubscriber/LbBlockDecoratorRouteSubscriber.php

+<?php
+
+namespace Drupal\lb_plus_lb_block_decorator\EventSubscriber;
+
+use Drupal\Core\Routing\RouteSubscriberBase;
+use Drupal\lb_plus_lb_block_decorator\Form\BlockDecoratorForm;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Route subscriber.
+ */
+final class LbBlockDecoratorRouteSubscriber extends RouteSubscriberBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function alterRoutes(RouteCollection $collection): void {
+    // Add support for nested layouts.
+    $route = $collection->get('lb_block_decorator.decorate');
+    $route->setDefault('_form', BlockDecoratorForm::class);
+    $route->setDefault('_title_callback', BlockDecoratorForm::class . '::title');
+    $route->setDefault('nested_storage_path', NULL);
+    $route->setPath($route->getPath() . '/{nested_storage_path}');
+  }
+
+}