New cross-platform WebView package: react-native-webview

Hey there! Thanks for your hard work on this package.

I've started work [extracting the built-in WebView](github.com/react-native-community/discussions-and-proposals/pull/3) into [its own package](https://github.com/react-native-community/react-native-webview). This package supports WKWebView (based on [Facebook's internal work](https://facebook.github.io/react-native/blog/2018/08/27/wkwebview)), Android, and (eventually) Windows 10 via react-native-windows.

After we get the package stable, our goal will be to improve support on all three platforms, including some of the things you've run into with this package.

My question is whether you'd like to collaborate on this new package. I don't intend to put any pressure on you or anything, just offering to combine efforts under the React Native Community repository.

Happy to answer questions and hear comments. Thank you!