Xamarin Forms support

[Xamarin Forms](https://www.xamarin.com/forms)
(Reference: https://github.com/ForNeVeR/wpf-math/issues/17#issuecomment-362284365)

Xamarin Forms is a cross-platform UI library that focuses on mobile platforms (iOS, Android, UWP), which got extended to desktop platforms very recently (Mac - UIKit, Linux - GTK#, Windows - WPF).
It is better than Avalonia in that with the 2016 Microsoft acquisition of Xamarin, this has become Microsoft-supported and got a huge boost.
As with Avalonia, this is an XAML-and-MVVM UI library too.

Btw, a C# LaTeX library has recently been ported to Xamarin.iOS (might worth a look): https://github.com/verybadcat/CSharpMath (It has died down a bit in the recent weeks though)