This will help downstreams like JupyterLite configure how they open paths more easily, without having to redefine all the plugins.

Paths can be:

• files, like example.py or notebooks/example.ipynb
• terminals
• consoles

By default in Notebook 7 paths are opened are joined with /, for example https://localhost:8888/notebooks/example.ipynb.
In the case of JupyterLite paths are opened by specifying the ?path= query string parameter in the URL, for example: https://localhost:8000/notebooks?path=example.ipynb`.

To avoid having downstream applications like JupyterLite have to override several plugins to be able to customize the behavior of window.open, the path opening logic is moved to a dedicated plugin so only that plugin would need to be redefined in other applications.

• Add INotebookPathOpener to be able to override the URL for opening paths
• Keep it optional to not break existing behavior in potential downstream applications reusing some notebook plugins