refactor(controllers): Refactored how Controllers interact w/ three

[saitonakamura]

This a big refactor of controller and how they interact with three counterparts. The flow of events was very confusing, to say the least. This PR makes this flow much direct and does some other notable changes.

ControllerModel abstraction is removed

It's responsibilities are moved to Controllers, mainly to https://github.com/pmndrs/react-xr/pull/294/files#diff-f17a5c6c1d801f88dd02bbbcaacc32ee99c5630535d1db5f3d5527ddd84b89eaR85-R103

And Controllers now directly render XRControllerModel

So the flow now is

1. XR creates a pair of XRController and subscribes to connected/disconnected events from three.js
2. XRController call xr.getControler and others in constructor therefore creating a WebXRController instances in three.js. Both WebXRController and it's XRController counterparts will now be present for the lifetime of an app
3. three.js WebXRManager react to inputsourceschange and populates it's state and dispatches connected event
4. connected event is handled by XR and it populates state.controller with corresponding XRController
5. handleControllerModel react to ref and calls modelFactory.initializeControllerModel which does all the 3d/profiles heavy lifting as previous

Previously, the 5th step was actually another multi-step flow which involved sending fake events and such

XRControllerModel moved to a separate file from XRControllersModelFactory

No notable changes inside it

InputSource on XRController can now be null

Now inputSource will become null if controller is disconnected, no real reason to hold on to outdated inputSource. This resulted in a bunch of null checks here and there, but nothing substantial

Whole bunch of unit tests added

We're still far from good coverage, but this is a big step forward

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[saitonakamura]

@CodyJasonBennett I'd really appreciate any feedback on this

[CodyJasonBennett]

I'm happy with the premise. Will give this a deeper review later today.