Add backward compatibility for ipywidgets 7

[martinRenou]

Closes #1487

Closes #1394 as this PR takes the approach of extracting the widget manager package out of the Voila one too.

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How?

Voila now ships two new labextensions:

• @voila-dashboards/widgets-manager7 for dealing with ipywidgets 7
• @voila-dashboards/widgets-manager8 for dealing with ipywidgets 8+

The server will check which version of @jupyter-widgets/jupyterlab-manager is already installed there, and decide to inject either one or the other Voila widget manager in the page.

---

[github-actions]

👈 Launch a Binder on branch martinRenou/voila/widgets_managers

[martinRenou]

Damn, I did not expect the UI-tests to pass 😆

[jtpio]

Could this have used the import type approach too?

[martinRenou]

Ah indeed I missed that one

[dfguerrerom]

my page_config_hook doesn't work anymore after this PR....

my previous logic was only adding the labextensions that were available in the virtual environment that I wanted to use... now I have refactored to also include the voila extensions... however, there's nothing rendered in my voila page.

my federated extensions are:

        "federated_extensions": [
            {
                "name": "jupyterlab-plotly",
                "load": "static/remoteEntry.41e12212782f05d7d17b.js",
                "extension": "./extension",
                "mimeExtension": "./mimeExtension",
                "entrypoints": None,
            },
            {
                "name": "jupyter-vue",
                "load": "static/remoteEntry.f993df579401788f32ef.js",
                "extension": "./extension",
                "entrypoints": None,
            },
            {
                "name": "jupyter-vuetify",
                "load": "static/remoteEntry.cba680c7ed013f0591de.js",
                "extension": "./extension",
                "entrypoints": None,
            },
            {
                "name": "jupyter-leaflet",
                "load": "static/remoteEntry.a83217134ba8dacb85fc.js",
                "extension": "./extension",
                "entrypoints": None,
            },
            {
                "name": "jupyterlab_pygments",
                "load": "static/remoteEntry.5cbb9d2323598fbda535.js",
                "extension": "./extension",
                "style": "./style",
                "entrypoints": None,
            },
            {
                "name": "@jupyter-widgets/jupyterlab-manager",
                "load": "static/remoteEntry.e4ff09401a2f575928c0.js",
                "extension": "./extension",
                "entrypoints": None,
            },
            {
                "name": "@jupyterhub/jupyter-server-proxy",
                "load": "static/remoteEntry.569b1096d01998b85e34.js",
                "extension": "./extension",
                "entrypoints": None,
            },
            {
                "name": "@voila-dashboards/widgets-manager8",
                "load": "static/remoteEntry.958dac8c7410b5fcc9ee.js",
                "extension": "./extension",
                "entrypoints": None,
            },
        ],

Is there anything else I've been missing?