Export LabWidgetManager and KernelWidgetManager

[jtpio]

This is a proposal to export the LabWidgetManager and KernelWidgetManager from the @jupyter-widgets/jupyterlab-manager package.

This will let third-party packages and extensions import these managers with:

import { KernelWidgetManager, LabWidgetManager } from '@jupyter-widgets/jupyterlab-manager';

Instead of:

import { KernelWidgetManager, LabWidgetManager } from '@jupyter-widgets/jupyterlab-manager/lib/manager';

This would be useful to other applications that would like to reuse the widget managers as is, or extend their functionalities by subclassing them. See voila-dashboards/voila#846 for a work-in-progress example.

Opening this PR as a way to discuss whether or not we should do this, since exposing new classes increases the API surface.

[github-actions]

👈 Launch a binder notebook on branch jtpio/ipywidgets/expose-widget-managers

[jtpio]

Adding to the 8.0 milestone for the API surface considerations.

[vidartf]

I'm 👍 on this.