Add stub for AbstractType

[gndk]

This PR adds stubs for Symfony\Component\Form\AbstractType and Symfony\Component\Form\AbstractTypeExtension.

After upgrading to Psalm 5, it requires @template-extends AbstractType<int> annotation when extending AbstractType, but currently phpstan complains about this.

Adding these stubs creates parity with psalm/psalm-plugin-symfony regarding those classes.

[stof]

what is expected to be represented by that template type ?

[gndk]

The stubs in the psalm plugin were originally added in this PR psalm/psalm-plugin-symfony#138, with some discussion here psalm/psalm-plugin-symfony#60.

[gndk]

There is also discussion around this current issue after upgrading to psalm 5 here psalm/psalm-plugin-symfony#294

[VincentLanglet]

Those stubs are useless for PHPStan and even more they will just add more pain for PHPStan-Symfony user because it will require to fill a template

/**
 * @extends AbstractType<ICanWriteEverythingIWant>
 */
class MyType extends AbstractType
{
}

With no benefit at all.

@stof Those stubs were added to psalm-symfony with only one benefit, this one:
https://github.com/psalm/psalm-plugin-symfony/blob/4.x/src/Stubs/common/Component/Form/FormView.stubphp#L16
but the syntax array{value: ?T, attr: array<array-key, mixed>}&array<string, mixed>
is not supported by PHPStan.

So there is absolutely no reason to add such template.

Also the symfony-core was not really into those generics symfony/symfony#40783 (comment)

If you want to use both psalm and phpstan, I assume you can write

/** 
 * @psalm-param FormView<Foo>
 * @phpstan-param FormView
 */