Register additional editor overrides when adding new notebook file types (#17708)

* Also standardized file extension contributions to always start with a period, and to always do lower case string comparisons for file extensions.

src/vs/workbench/test/browser/api/mainThreadDocumentsAndEditors.test.ts

@@ -103,7 +103,7 @@ suite('MainThreadDocumentsAndEditors', () => {
 			},
 			new TestPathService(),
 			<INotebookService>{ // {{SQL CARBON EDIT}}
-				getSupportedFileExtensions: () => ['ipynb']
+				getSupportedFileExtensions: () => ['.ipynb']
 			}
 		);
 	});