ReactNativeIsland should be accessible from the ReactNativeWin32App

[acoates-ms]

There are use cases where Apps may need access to the ReactNativeIsland object.

See the issue originally posted by @austinhallock in #14136 (comment)

[chrisglein]

Great to get feedback on the builder. Goal is that people can scale up to their needs from the basic things. This was something we missed exposing. Should be clear to add, let's try for 0.77. Just need a getter, not to provide custom island.

Design note: add to React Native Win32 app, not the builder. That's not the builder's job.
Needs to be accessible in the right timing window. It's creating in start, which is the loop and doesn't return. So there's some timing work to be thought through.