Set framerate to VR headset framerate

[TwistedTwigleg]

As mentioned by @aaronfranke on the Godot Documentation repository:

Also, instead of hard-coding the target framerate and physics framerate to 90, is there a way to detect the headset's framerate? For example, the Oculus Quest is 72 Hz, the Rift S is 80 Hz, and the Valve Index can do either 120 Hz or 144 Hz.

I do not know if it is possible to get the framerate of the VR headset right now, but if this is possible, or becomes a feature in the future, then it would probably be a good idea to dynamically set the framerate rather than use a hard-coded value.