Investigate decoupling OmniSharp plugin from Razor tooling

[davidwengier]

What does it take to allow Razor to continue to be hostable by Omnisharp, but not tied to internal Razor APIs.

[ryanbrandenburg]

Here's my thinking. Curious to hear if there are things which I didn't consider or that people want more details on.

(This was all wrong so I edited it).

[ryanbrandenburg]

(Seems I didn't quite follow the plot here, the above will likely change significantly)

[ryanbrandenburg]

Now that #7936 has merged the next steps here would be to create an ExternalAccess assembly and encapsulate the types that are needed (those which are still public after #7936) there.