Add Windows XamlWebView backend

[matthewbauer]

With @angerman's recent work on cross compiling GHC to windows (NixOS/nixpkgs#43559), it will hopefully be possible soon to build for Windows on Nix! If we can get a proper backend for jsaddle ready, this could mean building whole apps for Windows.

Originally I was thinking we can reuse the webkitgtk backend. However, Windows is no longer supported by webkitgtk, apparently (https://github.com/Alexpux/MINGW-packages/issues/3484). Instead, I am thinking it would make sense to go the similar route to WKWebView and reuse the OS SDK as much as possible. Microsoft provides WebView that we can make c bindings to:

https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.webview

I think these are usable in the Mingw based environment.