Expose pre-collision moving velocity to VehicleBlockCollisionEvent

[SoSeDiK]

Upon firing VehicleBlockCollisionEvent the collision already happened (for example, a minecart would already be stopped), and before there was no easy way to know the velocity at which this happened; you'd have to track that yourself.

Now you can use this API to implement features like bouncing from slime blocks depending on the speed you hit them :)