Extend watch to rebuild labextension

[jenshnielsen]

Without this I find reloading the labextension as described in the readme does not work. Adding this and it seems to work correctly. I have taken this dirrectly from the settings in ipympl https://github.com/matplotlib/ipympl/blob/master/package.json#L34

And set webpack in development mode to avoid a warning about mode being unset when watching

[github-actions[bot]]

:point_left: Launch a binder notebook on branch _jenshnielsen/ipycytoscape/watch_rebuild_labextension

[ianhi]

Thanks!