Compile option to remove the physics engine

[Swarkin]

or the option to replace it with a Dummy implementation.

I think this would be very helpful for non-game applications made in Godot to reduce both the .exe size and the needed performance.
I often use godot for non-gamedev things because I really like its UI system, so features like this would benefit a lot for this use case.

[Swarkin]

Fixed in godotengine/godot#95252, the 2D counterpart is still pending.

[Swarkin]

Both were merged in 4.4!