Add prepublish step to widgetsnbextension.
Add prepublish step to widgetsnbextension.

This makes sure we get the correct coordinated version of the html-manager package for the embed menu item.
jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets