Add PHPStan configuration for ConfigProcessor::process

[ruudk]

This way, PHPStan knows that input becomes output.

Example, not PHPStan understands this:

/**
 * @phpstan-import-type ObjectConfig from ObjectType
 */
final class MYType extends ObjectType implements AliasedInterface
{
    public function __construct(ConfigProcessor $processor, GraphQLServiceLocator $locator)
    {
        /**
         * @var ObjectConfig $config
         */
        $config = [
            'name' => self::NAME,
            'fields' => fn () => [
                'id' => [
                    'type' => Type::nonNull(Type::id()),
                ],
            ],
            'interfaces' => fn () => [
                $locator->getType(NodeType::class),
            ],
        ];
    
        parent::__construct($processor->process($config));
    }
}