Support Xamarin projects

[samsmithnz]

The project file metadata isn't consistent, needs some research. Involves TargetFrameworkVersion AND TargetFrameworkIdentifier - but not for every project type

• https://github.com/corbob/NuGetPackageExplorer/blob/60be2dfacd8ef5ca09f11b73bdb94189fdcedbca/PackageExplorer/PackageViewer.xaml.cs#L38
• https://github.com/xamarin/xamarin-forms-samples