VSLanguageService -> FSharp.Editing

[cloudRoutine]

Dependent upon completion of Open Documents Tracker #1410

Much of the functionality of the VSLanguageService can be moved to the LanguageService

• unsure how to address buildQueryLexState, to start ignore the conditional branch that requires the ITextBuffer
• EntityCache - Can this be re-implemented using a ConcurrentDictionary without the need for lock?
• use FCSPoint & ranges instead of SnapshotPoint and SnapshotSpan
  *instead of passing the ITextView in GetLoadDirectiveFileNameAtCursor pass the caret position directly
• unclear how to migrate and reportProgress from with FindUsages while in Core, save for end

Code Located @
https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/src/FSharp.Editing.VisualStudio/ProjectSystem/VSLanguageService.fs
https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/src/FSharp.Editing/ProjectSystem/LanguageService.fs

[vasily-kirichenko]

I don't think LanguageService should accumulate more functionality. In fact, I'd like make it lighter. Feature specific code should be spread among other types.