Use InputSystem.onBeforeUpdate instead of manually hooking into player loop

[TheNathannator]

The input system package provides its own update callbacks that can be hooked into for updating state. These callbacks are called in both play mode and edit mode, and using onBeforeUpdate guarantees that inputs are queued and processed in the same update (instead of being delayed for a frame).

[keijiro]

Thanks for the PR. I had realized this approach recently as well but couldn't afford the time to update the code.

I'll run some tests, and if everything looks good, I'll merge it.