Refactor / p5xr consolidation / Fix AR mode

[TiborUdvari]

As discussed in #207, this PR consolidates common code between p5xr and p5ar into p5xr in preparation for a more feature based approach. Initially I was looking to fix AR mode for the use with my Quest 3, but doing the refactor at the same time made the most sense.

Let me know if you would like me to squash the commits into a single one, or if you prefer "atomic commits".

Here are the changes in reverse chronological order:

• Hello cube AR example
• Fix tests for launching from constructor
• Consolidated common startsketch and request session into p5xr Removed p5.instance._decrementPreload as it was launching p5 in 2D mode for some reason
• Moved common requestSession code into p5xr from p5ar and p5vr
• Favour explicited WebXR modes
• Moved isImmersive to XR level, AR and VR can both be immersive
• Removed superfluous init functions, create buttons in constructors Removed button argument because it was always created and overloaded afer construction
• Removed deprecated navigator.getUserMedia() https://developer.mozilla.org/en-US/docs/Web/API/Navigator/getUserMedia

[stalgiag]

This looks exciting! I just read through the code and everything makes sense. Later this month I can get my hands on a headset for testing but let me know if you would prefer a more expedited timeline. I would also be open to the idea of promoting your permissions and allowing you to merge sooner and I can follow up with bug reports if something is amiss once I am able to actually test.

[TiborUdvari]

Thanks! There is this Immersive Web Emulator which is nice for testing.

I think I prefer to merge once you could check things. I setup npm publishing from my fork as well, I think it's useful for testing out features where it's unclear what the p5 like api should be, like in #216

Since there don't seem to be any other active contributors at the moment I think the risk of conflicts is pretty minimal.

So better keep it as is for now IMO.