react-native-xaml support for Fabric

[asklar]

Once we have both a working hybrid library template for RNW, and the requisite Xaml Island support, we will need to upgrade RNX with new code-gen that works in Fabric.

### Tasks
- [x] Prototype a version of react-native-xaml-fabric to prove out the fabric-based version.
- [ ] Design: decide where the XamlIsland boundary will be.
- [ ] Design: decide how to manage the Application object and how components can add metadata.
- [ ] Get some basic controls running without codegen.
- [ ] Get codegen working to the 80% case.
- [ ] Test out the top 10 most interesting controls.
- [ ] Make the XamlIsland do the right thing at high-DPI.
- [ ] Merge react-native-xaml-fabric into react-native-xaml, allowing NewArch apps to pick up the fabric logic and other apps to keep picking up the paper logic.