Fix/prev next links issue (#10523)

* intital test code

* added tests

* remove commented code

* fix for failing tests

* reuse exported enum

* changes to address comments

* add back onVisibility highlight

* port highlight fix from Chris

* fix tests

src/sql/workbench/services/notebook/browser/notebookService.ts

@@ -34,6 +34,16 @@ export interface ILanguageMagic {
 	executionTarget?: string;
 }
 
+/**
+*  Valid navigation providers.
+*/
+export enum NavigationProviders {
+	NotebooksNavigator = 'BookNavigator.Notebooks',
+	ProvidedBooksNavigator = 'BookNavigator.ProvidedBooks'
+}
+
+export const unsavedBooksContextKey = 'unsavedBooks';
+
 export interface INotebookService {
 	_serviceBrand: undefined;