Create a XAML language service

It would be awesome to start working towards a proper XAML editor in VS Code.

A few things off the top of my head that this would need to accomplish:
1. Provide intellisense when user is editing xaml files
2. Provide syntax highlighting
3. Not be tied to a particular XAML dialect, and be extensible so we can build UWP/WPF/Xamarin.Forms/Avalonia experiences inside VS Code on top of it.

I'm not entirely familiar with how a VS code language service operates. I've only very briefly dabbled with them a few months ago, but I feel like the Omnisharp project is the right place for this to reside.

/cc @kekekeks @michael-hawker @danwalmsley @grokys