HTMLManager: Support both ipywidgets 7 and 8 models

[martinRenou]

Currently, the HTML manager brings the jupyter-widgets front-end for ipywidgets 8, disregarding the version stored in the widget model.
This works fine in most cases, as long as the user did not store an ipywidgets 7 model that was making use of removed APIs.

This PR resolves this by lazily loading either the jupyter-widgets front-end for 7 or 8 depending on the version stored in the model. Similar to the approach taken in voila-dashboards/voila#1482

Testing nbconvert output with ipywidgets 7:

Testing nbconvert output with ipywidgets 8:

[github-actions]

👈 Launch a binder notebook on branch martinRenou/ipywidgets/html_manager_7_8

[SylvainCorlay]

👍

[SylvainCorlay]

I like this solution. It will solve many user's headaches.

[maartenbreddels]

Nice work! In solara we use a similar trick with the npm alias, but we actually have to build 2 separate managers. I think this is a cleaner solution.

[martinRenou]

Nice work! In solara we use a similar trick with the npm alias, but we actually have to build 2 separate managers. I think this is a cleaner solution.

Oh nice I did not know you tried that in Solara too! Note that we've done something similar in Voila 0.4.x but with just one manager: voila-dashboards/voila@5934c53, it's a bit different from the approach in this PR as it makes use of the lazy loading approach in the widgets register, so the widgets assets will load only upon request.

I tried with latest Voila but things are more complicated there for some reason.

[maartenbreddels]

In solara (because we run in the same process), we know beforehand at the server what the version of ipywidgets is, which allows for a different strategy.
We also build the final bundle twice using the webpack alias trick:
https://github.com/widgetti/solara/blob/20b2e8012e7683eb38ff62a74a371eb1a052bf41/packages/solara-vuetify-app/webpack.config.js#L95

I don't think you need it here, but both these tricks allow for some fancy building/bundling options.

[martinRenou]

We also build the final bundle twice using the webpack alias trick:
https://github.com/widgetti/solara/blob/20b2e8012e7683eb38ff62a74a371eb1a052bf41/packages/solara-vuetify-app/webpack.config.js#L95

Ah, that trick could probably be useful to me for Voila 0.5.x, I'll have to give it a try. Thanks for sharing!