Make `ARSwiftUIView.proxy` non-optional

[philium]

ARSwiftUIView's initializer isn't ever called without a proxy object.

[philium]

Any downside to making proxy internal and using a synthesized initializer instead?

The only downside is that the doc comment goes away. That's why I left it. But if you're fine with that, I'm fine with doing it.

[philium]

I'll leave it as is since the compiler synthesized implementation would include all of the other properties as parameters as well.