Add support for custom screenshot drivers

[stevebauman]

I've added a Laravel-style driver manager so you can register custom capture drivers with Screenshot::extend() and select them with Screenshot::driver('remote'). Chrome remains the default, and existing fluent calls continue to use the configured driver.

Drivers share the same capture builders and defaults. Screenshot::fake() records calls across all driver names, and both the manager and fake implement Contracts\Factory so you can use constructor injection in application code.

This changes the container API: inject Contracts\Factory instead of the concrete Screenshot class, and register custom drivers through extend() instead of replacing the Driver binding.