Symfony 4.2: Support improved form type extensions

[Haehnchen]

Dont not lose support for form extension following form extension must be added. First one already included by we need also the iterable+static behavior

    public function getExtendedType()
    {
        return FileType::class;
    }

class ImageTypeExtension extends AbstractTypeExtension
{
    // ...

    public static function getExtendedTypes(): iterable
    {
        return [FileType::class];
    }
}

    public static function getExtendedTypes(): iterable
    {
        yield DateTimeType::class;
        yield DateType::class;
        yield TimeType::class;
    }

symfony/symfony#24530
https://symfony.com/blog/new-in-symfony-4-2-improved-form-type-extensions