Add React Native for Windows vNext support

[stmoy]

Feature Request

React Native for Windows enables React Native apps to target Windows via UWP. React Native for Windows is under active development by Microsoft. Most recently, the platform has been updated to leverage high-performance C++ (known as "RNW vNext"). Now that the RNW vNext supports native modules as a platform, we have identified several key native modules to add Windows support for (including react-native-video) to help validate that the platform has the right set of capabilities and to bootstrap the community of modules that target Windows.

Microsoft devs are signed up to help build out this implementation, but we wanted to track this work here as well to help provide some transparency that we're doing this work and hoping to contribute back to the repo.

(Note that this is also a topic of conversation on #1628)

Why it is needed

Microsoft devs are signed up to help build out this implementation, but we wanted to track this work here as well to help provide some transparency that we're doing this work and hoping to contribute back to the repo.

Possible implementation

This implementation will use the updated React Native for Windows vNext native module infrastructure, documented here: https://github.com/microsoft/react-native-windows/blob/master/vnext/docs/NativeModules.md

On the react-native-windows GitHub, we're tracking this work with the following issue:
microsoft/react-native-windows#2095

Code sample