Port Hover endpoint to cohosting

[DustinCampbell]

Moving Hover to co-hosting presents a few challenges.

1. Hover can produce either MarkupContent or, if Visual Studio is the client, a ContainerElement from the Microsoft.VisualStudio.Text.Adornments namespace. For Roslyn, the production of ContainerElement happens in the Editor Features layer, which is not part of Roslyn OOP. So, some re-layering in Roslyn will be necessary.
2. In multi-targeting scenarios, Hover will add text that describes which targets a tag helper or component is not available in. We'll need an API that can resolve a document file path to the projects that contains it.
3. The hover infrastructure is leveraged by completion to for completion item tooltips. So, Port Completion endpoints to cohosting #10697 is dependent on work that'll happen as part of this issue.

There's also some related work that I'd like to get try and tackle:

1. Hover for Razor tag helpers and components has very sketchy code that uses regex's to try and "parse" XML doc comments. We should add a facility to the Razor EA to that parses XML doc comments using Roslyn.