Provide a custom `IDocumentManager`

[jtpio]

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Fixes #6344

This avoids having to patch docManager.open method at runtime.

However since the IDocumentManager.IWidgetOpener requires access to the doc manager to handle dirty contexts, some logic is similar to the one in the @jupyterlab/docmanager-extension:manager plugin.

[github-actions[bot]]

:point_left: Launch a Binder on branch jtpio/notebook/doc-manager