Fixes #873

Sometimes there are buffers with our C#/VB content types that are not added to the Running Document Table and therefore never end up in the MiscellaneousFilesWorkspace (or any workspace), such as the Source Control "Annotate" view. This change updates the Syntactic Classifier to handle such cases by parsing its initial snapshot for tagging (changes are not observed, but the known use cases of this fix are uneditable anyway).  There are some exceptions to this behavior (e.g. the Immediate window) which require us to go digging through attached views, but we default to classifying these buffers now.

since this logic is now exposed to outside of engine, engine needs to be guarded from abuser so, logic is added to drop cache if there is too many high priority work. before only engine can decide what is high priority or not.

When a project or document is removed/closed, clear out all the squiggles we currently have for them.

This affects TS as they routlinely close/open projects behind the scenes, while still using the
current editor buffer.

* Remove unintentionally added extra option.
* Rename NavInfo property to NavInfoFactory.
* Add fallback to metadata-as-source if NavInfo can't be retrieved for some reason.
* Use [ExportLanguageService] attributes rather than language service factories.
* Add NavInfo tests for generic methods and types
* Add negative NavInfo tests for parameters, locals and labels
* Rename UsePreviewTab option to PreferProvisionalTab to remove some confusion around what it does, and document it.

1. Ensure the destination is visible and not collapsed.
2. Disable in contained documents (as in VS2013)
3. Handle being outside any method level member
4. Add tests for VB.
5. Add tests for script.

Conflicts:
	src/EditorFeatures/Core/EditorFeatures.csproj