Changes in this PR:

1. Add HintedSuggestion to XamlDiagnosticSeverity and handle this new severity in AbstractPullDiagnosticHandler.

What is HintedSuggestion?
HintedSuggestion is a new error Severity XAML added lately to support errors that show the "..." marker in editor but don't show up in the error list.

2. Switch to listening to Workspace.DocumentClosed event instead of RDT OnBeforeLastDocumentUnlock in XamlProjectService. RunningDocumentTableEventTracker is also listening to OnBeforeLastDocumentUnlock and will fire a DocumentClosed event for it. This OnBeforeLastDocumentUnlock inside RunningDocumentTableEventTracker is doing the same check as we have in XamlProjectService. There is no need for us to do duplicate work and meanwhile as we remove the XAML file when the file gets closed this will ensure the file only gets removed after the DocumentClosed event gets fired.