Improve C# experience with analyzer and source generator

[chkn]

This introduces a C# source generator to bring a couple improvements:

• The Body property can be declared simply as View instead of the exact derived view type that it returns.
• An overload may have an Action parameter marked with the [ViewBuilder] attribute. Inside the Body property, the lambda argument will be transformed using the same transformation the Swift compiler uses. This will enable a natural syntax for container views.

There is also an accompanying analyzer to enforce a couple policies:

• Custom views must declare a Body property
• Custom views must be declared partial (this is required for the aforementioned source generator to work)

This also adds a NuGet dependency to automatically use a new enough build of the Roslyn compiler to enable the source generator in currently shipping Xamarin.iOS and Xamarin.Mac projects. However, if the system mono doesn't also contain a new enough build of Roslyn, the code fixes for the above analyses won't be available in VS Mac.

Still TODO:

• [x] Enable the new analyzer tests on CI

[CartBlanche]

Hoping to play with this by the end of the week.