Fix microsoft#229883. content event is required for cell content change. (microsoft#229896)

Author: rebornix

src/vs/workbench/contrib/notebook/common/model/notebookCellTextModel.ts

@@ -160,6 +160,7 @@ export class NotebookCellTextModel extends Disposable implements ICell {
 					this._alternativeId = this._textModel.getAlternativeVersionId();
 				}
 				this._textBufferHash = null;
+				this._onDidChangeContent.fire('content');
 				this._onDidChangeContent.fire({ type: 'model', event: e });
 			}));