feat: stub DataMapperInterface

[roerbakei]

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

I've set TData of FormInterface to mixed since each child form can have a different type.

refs: #417

[JefvdA]

As it's stated in the docs ( https://symfony.com/doc/current/form/data_mappers.html#creating-a-data-mapper ), the following logic is needed:

// there is no data yet, so nothing to prepopulate
if (null === $viewData) {
    return;
}

This, as it seems that mapDataToForms is called before the data is avaialble. Which will mean that $viewData will be null at that time.

---

Thus I propose the following change:

Suggested change

	* @param TViewData $viewData
	* @param \Traversable<mixed, FormInterface<mixed>> $forms
	*/
	public function mapDataToForms(mixed $viewData, \Traversable $forms): void;
	/**
	* @param \Traversable<mixed, FormInterface<mixed>> $forms
	* @param TViewData $viewData
	* @param-out TViewData $viewData
	*/
	public function mapFormsToData(\Traversable $forms, mixed &$viewData): void;
	* @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;

---

For example, the stub file for the FormTypeInterface also has a generic type, but also has it defined nullable when it's being passed to it's buildForm method.

[ondrejmirtes]

There are two different PRs with this stub: #422

1. Both have the same mistake of not registering the stub.
2. Both are slightly different so you should agree first how it should look like.