VisionOS Support

[robertherber]

Description

I just gave VisionOS a quick try, and I guess it's something that would be nice to have supported :)

1. First thing I ran into was that the Podfile doesn't get installed, which was easily resolved by updating the react-native-skia.podspec by adding visionOS together with ios:
   s.platforms = { :ios => "13.0", :visionos => "1.0"}
2. The next thing I ran into is 'UIScreen' is unavailable: not available on visionOS in RNSKiOSPlatformContext.h:36. I fixed this by hard-coding 3.0 since it's just a scale
3. Now I'm running into a potentially bigger issue. Seems like GLKit is not available on VisionOS. I tried removing it from the podspec, but that gives me lots of missing symbols. I see the podspec also includes MetalKit which I guess is the proposed successor of GLKit - but I guess skia depends on both or could there be a way around this?

I have no idea how deep this hole goes, but if it's not too difficult I'd appreciate some pointers on next steps, and would be happy to test out and potentially make some PRs for adding VisionOS support.