Resolve #3479141 "Implement FormAlter attribute"

Closes #3479141

core/lib/Drupal/Core/Hook/Attribute/FormAlter.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Core\Hook\Attribute;
+
+/**
+ * Hook attribute for FormAlter.
+ *
+ * @see hook_form_alter().
+ */
+#[\Attribute(\Attribute::TARGET_CLASS | \Attribute::TARGET_METHOD | \Attribute::IS_REPEATABLE)]
+class FormAlter extends Hook {
+
+  /**
+   * {@inheritdoc}
+   */
+  public const string PREFIX = 'form';
+
+  /**
+   * {@inheritdoc}
+   */
+  public const string SUFFIX = 'alter';
+
+  /**
+   * Constructs a FormAlter attribute object.
+   *
+   * @param string $form_id
+   *   (optional) The ID of the form that this implementation alters.
+   *   If this is left blank then `form_alter` is the hook that is registered.
+   * @param string $method
+   *   (optional) The method name. If this attribute is on a method, this
+   *   parameter is not required. If this attribute is on a class and this
+   *   parameter is omitted, the class must have an __invoke() method, which is
+   *   taken as the hook implementation.
+   * @param string|null $module
+   *   (optional) The module this implementation is for. This allows one module
+   *   to implement a hook on behalf of another module. Defaults to the module
+   *   the implementation is in.
+   */
+  public function __construct(
+    string $form_id = '',
+    public string $method = '',
+    public ?string $module = NULL,
+  ) {
+    parent::__construct($form_id, $method, $module);
+  }
+
+}