[codemirror] prevent default if key down handler returns true
[codemirror] prevent default if key down handler returns true

charnpreetsingh185/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab