editor: add undo stop when switching model

Since we share the same editor instance with multiple models, we need
to ensure undo stack consistency when changing the model, otherwise
we can lose some history, e.g. if a not active model is modified.

src/editor/sagas.ts

@@ -265,8 +265,10 @@ function* handleEditorActivateFile(
             );
         }
 
+        editor.pushUndoStop();
         editor.setModel(file.model);
         editor.restoreViewState(file.viewState);
+        editor.pushUndoStop();
         activeFileHistory.push(action.uuid);
 
         editor.focus();