Provide a way for consumers to validate a VS instance

[DustinCampbell]

OmniSharp ships a very similar MSBuild discovery mechanism to this one. However, it verifies that the VS instance that it considers has the "Microsoft.VisualStudio.Components.Roslyn.Compiler" package installed.

https://github.com/OmniSharp/omnisharp-roslyn/blob/master/src/OmniSharp.Host/MSBuild/Discovery/Providers/VisualStudioInstanceProvider.cs#L56-L57

It would be great to provide consumers with some way to validate an instance in a similar manner. Otherwise, it's difficult to determine whether an instance is actually a reasonable choice.