[8.x] Use actual countable interface on MessageBag

[driesvints]

Instead of defining our own declaration of Countable, let's use the actual PHP interface. This is the only contract in Laravel that defines its own count method.

I've also removed the Arrayable interface from the MessageBag class because it's already defined through the MessageBag interface.

Ideally, both Arrayable and Countable wouldn't exist on the MessageBag interface but only on the concrete implementation.