Additional OpenXR-to-WebXR mapping package

[thetuvix]

We can drive further cross-UA conformance for UAs built atop OpenXR. Another package of JSON mappings could map each WebXR input profile to its corresponding OpenXR interaction profile, defining which button/axis index maps to which OpenXR /user path. OpenXR-based UAs could then simply merge in the latest OpenXR mappings once per release to inherit conformant support for all the latest controllers.

Originally posted by @thetuvix in #78

[NellWaliczek]

After talking with @thetuvix about the requirements, it sounds like this would be better as an extension to the existing registry schema rather than a separate package.

One other thing to consider is that there may be value in not special casing OpenXR in the schema, but instead including information about the mappings from each supported native platform.

I'll throw together a couple variants for how this might look and post them for feedback.

[NellWaliczek]

Sample idea is here: https://github.com/immersive-web/webxr-input-profiles/blob/open-xr/packages/registry/profiles/generic/generic-trigger-squeeze-touchpad.json

[toji]

I'd like to bump this to the next release, so that we can make the existing libraries and assets available faster.