Clean up LanguageServerFeatureOptions

[davidwengier]

At the moment this class is used as a grab bag of settings that may or may not come from feature flags, though that was its original intention. We should clean it up.

• When rzls is retired we can remove the Configurable version, and some VS Code specific items, like UpdateBuffersForClosedDocuments and DelegateToCSharpOnDiagnosticPublish
• Instead of using a flag to detect VS Code, like SupportsSoftSelectionInCompletion and UseVsCodeCompletionCommitCharacters, we can use ClientCapabilities which is more in tune with its purpose

Above list may not be complete