Enable react-native-windows support

[chrisfromwork]

We are starting on the process of enabling react-native-windows support. We will use this issue to track progress.

Some known requirements:

• Wrap react-native-windows jsi runtime with NAPI
• Obtain javascript thread for animation callback logic
• Implement BabylonReactNative native module components
• Create BabylonReactNative windows package with precompiled native module binaries

[chrisfromwork]

microsoft/react-native-windows#6582

This pull request should give us access to the react-native-windows jsi runtime. It's unclear what additional work will be needed to get things working with NAPI

[chrisfromwork]

Work is out of the following branch:
https://github.com/chrisfromwork/BabylonReactNative/tree/uwp

Building a local version of react-native-windows based on the following PR reveled some more issues:
microsoft/react-native-windows#6617

1. bx.lib (bgfx dependencies) consumes the windows runtime for threading logic. This conflicts with building winmd c++ winrt dlls. It seems fairly straight forward to use non-windows runtime (c++/cx) logic to setup threading. We may need to contribute some sort of c++ winrt friendly bx.lib definition to bgfx.
2. react-native-windows has a bug in JsiApi.cpp where the s_jsiRuntimeMap is not correctly populated outside of unit tests. This causes napi population of BabylonNative components to fail. This should be a straight forward fix, but may take some time to get back into the react-native-windows codebase

at this point, calling logic on both the ui and javascript thread in react-native-windows native module definition seems to work correctly

[chrisfromwork]

At this point we have a working prototype for UWP. There are a few dependencies we will be waiting on:

• 1. We need a react-native-windows npm package containing the following change: Fix getting JsiRuntime for host functions microsoft/react-native-windows#6644 This should appear in a canary release soon
• 2. We need some changes in bgfx's bx library: Add flag to avoid consuming windows runtime in winrt c++ applications bkaradzic/bx#259
• 3. We need to update BabylonNative bgfx cmake dependencies: Add a new cmake flag to avoid consuming windows runtime in static libs bgfx.cmake#20
• 4. Some BabylonNative fixes: Fixes for react native UWP support BabylonNative#549
• 5. We need BabylonNative to reference newer bgfx cmake submodule commits: Update BabylonNative to consume newer bgfx.cmake submodule commit BabylonNative#558