feat: added stub for DataMapperInterface

[JefvdA]

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: #417

[Tomsgu]

This looks good to me. The only missing part is detecting if I pass a correct value when creating a form. Because currently I can pass anything I want.

$form = $this->createForm(SnippetType::class, $notAllowedObject); // type mismatch error

 /**
 * @template-implements DataMapperInterface<SnippetDto>
 */
class SnippetType extends AbstractType implements DataMapperInterface
{
    public function mapDataToForms(mixed $viewData, Traversable $forms): void
    {
        if ($viewData === null) {
            return;
        }

        // Does not make sense to do this check. But we still need to do it. Obvious error: 
        // Instanceof between SnippetDto and SnippetDto will always evaluate to true.
        // 🪪  instanceof.alwaysTrue
        
        //if (!$viewData instanceof SnippetDto) {
        //    throw new UnexpectedTypeException($viewData, SnippetDto::class);
        //}

        $forms = iterator_to_array($forms);
        // ...
    }
}

I am wondering if there is anything we can do about this.