JupyterLab extension files not bundled into pip package

[mbektas]

we discovered this issue while working on documentation with @pbugnion. widget-cookiecutter is not including lab extension files into pip package unlike widget-ts-cookiecutter does in https://github.com/jupyter-widgets/widget-ts-cookiecutter/blob/master/%7B%7Bcookiecutter.github_project_name%7D%7D/setup.py#L49

[martinRenou]

I'd love to see this happen actually. This goes along with the discussion we had on ipywidgets jupyter-widgets/ipywidgets#2806. I've made this update in ipyleaflet and ipympl very recently. But it would be super nice if all widgets libraries do that, it would simplify things.

I can come up with a PR.