Add process_mode property to InterpolatedCamera

[saneery]

move_and_slide in _physics_process + InterpolatedCamera == stutter

Like this #24806 but for InterpolatedCamera
This allows the user to choose if the camera should update in _process or _physics_process.

[Calinou]

Note that integrating lawnjelly's smoothing-addon is a better way to combat stuttering compared to updating the camera position in the physics process loop.

[akien-mga]

Sorry for the delay reviewing this. I think this makes sense, and this is consistent with ClippedCamera and Camera2D (Camera itself doesn't have it, but doesn't seem to do any processing).

I force pushed a rebase to fix some style issues, add documentation, and properly initialize the process_mode property (to idle, to preserve compatibility with 3.4).

[akien-mga]

Thanks!