Add air_velocity for handling the players movement in the air.

[SirLich]

This is without feature flags (I removed some because I didn't need them), but splitting acceleration from when you're in the air Vs. on the ground is a good way to give more control:

	if is_on_floor():
		velocity.x = lerp(velocity.x, direction.x * speed, acceleration * delta)
		velocity.z = lerp(velocity.z, direction.z * speed, acceleration * delta)
	else:
		velocity.x = lerp(velocity.x, direction.x * speed, air_acceleration * delta)
		velocity.z = lerp(velocity.z, direction.z * speed, air_acceleration * delta)

I can submit a PR if you think this is a good addition (of course, it will still respect the existing flags for motion smoothing etc.