Fix undefined ModelAladin issue

[bmatthieu3]

This seems to work now on my python 3.8.9 What has been done:

• I changed the version of @jupyter-widgets/base to 6 in package json
• I updated the webpack.config.js so that it works with recent webpack version. There is now npm run build and prepublish commands that bundle the extension in ipyaladin/static (build for dev, prepublish for prod)
• We use modern js to define classes and export (I find it more readable)

This may be linked to: https://github.com/cds-astro/ipyaladin/issues/36

[bmatthieu3]

I merge that as it seems to work for python 3.8-x (checked on binder)