M4: Reconcile Microsoft.ReactNative and ReactUWP

[jonthysell]

Microsoft.ReactNative.dll is the new entry-point for React Native Windows developers. It is built on top of the "old" ReactUWP.dll.

The surface of ReactUWP.dll is a mix of standard exports with headers, an incomplete C++/WinRT winmd, etc.

The end goal is for Microsoft.ReactNative.dll to subsume and replace ReactUWP.dll, providing one, clean, WinRT entry-point. However there are existing partners shipping on top of ReactUWP.dll, so we need to make sure that they aren't left hanging until they can migrate.

[jonthysell]

For future reference, @vmoroz started a merge of the two DLLs here: https://github.com/vmoroz/react-native-windows/tree/ReactNativeHost/vnext/Microsoft.ReactNative

[jonthysell]

A huge chunk of this was done in #3809. Microsoft.ReactNative no longer depends on ReactUWP, but instead shares code with it, while we migrate customers off ReactUWP onto Microsoft.ReactNative.