Make the serverRoot variable available in DOM

[brichet]

This PR is a proposition to fix #6603

It make the serverRoot path available in #jupyter-config-data script when the option exposeAppInBrowser is True.

[github-actions]

👈 Launch a Binder on branch brichet/notebook/serverRoot

[jtpio]

Thanks @brichet.

I checked the JupyterLab (server) code and serverRoot does not seem to be exposed there: https://github.com/jupyterlab/jupyterlab_server/blob/904bd9535934f210ca270c9a43bf85afdfe7c3f2/jupyterlab_server/handlers.py#L89

Although it might be in another place.

What kind of testing requires accessing serverRoot?

[brichet]

@jtpio if I'm not wrong it is exposed here: https://github.com/jupyterlab/jupyterlab_server/blob/904bd9535934f210ca270c9a43bf85afdfe7c3f2/jupyterlab_server/handlers.py#L86
BTW, maybe it should be changed as it exposes the full path without any specific setting.

The serverRoot is used in UI tests (playwright) that requires to switch to the root directory of the application, to run some system command: https://github.com/jupyter/nbgrader/blob/066e793fb973b8ed03ea222a28c81950361edc94/nbgrader/tests/labextension_ui-tests/tests/test_utils.ts#L61
That was the easiest way I found to retrieve the root directory from UI tests, but there might be other ways.

[brichet]

Finally we found a work around with @fcollonval, using environment variables.
I close this PR, feel free to reopen it if the feature can be useful.

[jtpio]

Alright. For now I would say it's best to not expose more options to the client unless very necessary.