Xaml support prototype

[JesseCol]

Here's the idea:

• The react-native-windows package now contains a Microsoft.ReactNative.Xaml.dll to help with Xaml hosting.
• RNW exposes a type named XamlApplication in IDL to help coordinate Xaml work and metadata.
• RNW exposes a native component named XamlHost that knows how to host Xaml (using a XamlIsland). Native components that want to be Xaml-based can implement IXamlControl to allow their object to be parented to a XamlHost.
• There's a new package for testing, called xaml-calendar-view, which shows a sample implementation of a Xaml control.
• sample-app-fabric now uses a CalendarView from the xaml-calendar-view package.

Microsoft Reviewers: Open in CodeFlow