Support notebook file types contribution (#3196)

* Support notebook file types contribution
- Extensions can define a provider and what file types it should be used for
- Verified that this works for Jupyter Content & Server Managers.
- Starts Jupyter server as expected

Not in this PR:
- Support for session manager end to end
- Tests

src/sql/services/notebook/notebookService.ts

@@ -40,6 +40,8 @@ export interface INotebookService {
 	 */
 	getOrCreateNotebookManager(providerId: string, uri: URI): Thenable<INotebookManager>;
 
+	handleNotebookClosed(uri: URI): void;
+
 	shutdown(): void;
 
 	getMimeRegistry(): RenderMimeRegistry;