Save the full Voyager state to a `.vy.json` file

[fionc]

https://github.com/altair-viz/jupyterlab_voyager/issues/12

The other data output/save examples are focused on the saving a single chart. A user should also be able to save the state of the entire redux store as a file and reload it later. We would want to formalize the idea of a .vy.json file type