Allow Lua write access to camera_t variables & expose the cameras globally

I got sick of using HUD hooks and awayview objects to mess with cameras :sillychamp:

Exposes camera and camera2 as Lua globals, P_TryCameraMove and P_TeleportCameraMove functions, and allows write access to camera variables z, radius, height, momx, momy, momz, chase, angle, and aiming.

The following camera fields are also available as read-only: subsector, floorz, ceilingz, x, y

Will probably do a hook later, but will wait for !1577 (merged) to avoid conflicts.