UnusedSymbolClassifier -> FSharp.Editing

[cloudRoutine]

Dependent upon completion of VSLanguageService #1412

It should be possible to move part of the functionality in the classifier to Core, but this will require restructuring some of the functions.

• getOpenDeclarations can be moved without issue
• checkAstIsNotEmpty should probably become a utility function

updateUnusedSymbols will need some involved reworking to be based around documents on the core side and ranges instead of snapshots. The new function on the Core side should return a CategorizedSpan [] and a FSharpErrorInfo [] which the logic side can transform into the proper shape for VS.

I'm not sure if anything in onBufferChanged can or should be moved to Core

Code Located @ https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/src/FSharp.Editing.VisualStudio/Coloring/UnusedSymbolClassifier.fs