feat: added stub for DataMapperInterface

Ensure TData of FormInterface is provided
Set TData on mixed since we don't know the type of the child forms
Add template for  in DataMapperInterface

creds to @roerbakei ;)

refs: phpstan#417

stubs/Symfony/Component/Form/DataMapperInterface.stub

@@ -0,0 +1,22 @@
+<?php
+
+namespace Symfony\Component\Form;
+
+/**
+ * @template TViewData
+ */
+interface DataMapperInterface
+{
+    /**
+     * @param TViewData|null $viewData
+     * @param \Traversable<mixed, FormInterface<mixed>> $forms
+     */
+    public function mapDataToForms(mixed $viewData, \Traversable $forms): void;
+
+    /**
+     * @param \Traversable<mixed, FormInterface<mixed>> $forms
+     * @param TViewData|null $viewData
+     * @param-out TViewData $viewData
+     */
+    public function mapFormsToData(\Traversable $forms, mixed &$viewData): void;
+}