Split the JupyterLab extension into plugins that provide the widget manager and register widgets

[jasongrout-db]

This paves the way to having separate extensions that load multiple versions of core widgets, so that ipywidgets 7.x and 8.x can be supported simultaneously, for example.

See also conversation at #3429

[github-actions]

👈 Launch a binder notebook on branch jasongrout/ipywidgets/plugins

[vidartf]

@jasongrout What work remains here? Is this something that we should block 8.0 for?

[jasongrout]

@jasongrout What work remains here? Is this something that we should block 8.0 for?

I fixed the remaning errors, and I think this is ready for review.

[vidartf]

The code changes here are fine. Thinking about the stated goal of supporting 7.x and 8.x side-by-side, I do have some concerns w.r.t to most third party extensions declaring base and controls (if depending on controls) as singletons in their package.json's jupyterlab section.

[jasongrout]

I do have some concerns w.r.t to most third party extensions declaring base and controls (if depending on controls) as singletons in their package.json's jupyterlab section.

They should be doing that anyway.