Introduce form decorator attribute plugins

Closes #3501677

modules/form_decorator_example/src/FormDecorator/DependencyInjection.php

@@ -9,15 +9,13 @@ use Drupal\form_decorator\FormDecoratorBase;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\form_decorator\Attribute\FormDecorator;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
  * Adds dependency injection to the user login form.
- *
- * @FormDecorator(
- *   hook = "form_user_login_form_alter"
- * )
  */
+#[FormDecorator('form_user_login_form_alter')]
 final class DependencyInjection extends FormDecoratorBase implements ContainerFactoryPluginInterface {
 
   use StringTranslationTrait;