Add the option to open JupyterLite window in new tab

[melissawm]

My use case is the following: I'd like to use the jupyterlite directive to open the full page I am visiting as a Jupytext notebook. This PR adds that option to the JupyterLite directive.

Pending is finding the correct path for opening the current file as an ipynb file: since the source is an .md file, it will be converted into an .ipynb file at some point in the build process. For now, however, Sphinx can't process the directive since it will try to embed a link to the ipynb file before reading the current source .md file, and complain about the missing reference.

I am not sure if I am following the right path here so please let me know if I should approach this differently.

cc @agriyakhetarpal

[melissawm]

Here is an example of this working (partially): https://melissawm.github.io/scipy/tutorial/stats/sampling.html

[martinRenou]

Thanks for contributing!

It's deployed in the readthedocs preview: https://jupyterlite-sphinx--165.org.readthedocs.build/en/165/directives/jupyterlite.html

Changes look good to me. I was about to suggest doing the same for other directives, but when we implement https://github.com/jupyterlite/jupyterlite-sphinx/issues/114, we can make sure the :new_tab: parameter works well with the coming :app: parameter, so let's not bother about this in this PR?

Leaving this open to let other people have a look.

[melissawm]

I'm happy to do a follow up for that in the future, I might need some time togo over the app parameter and understand the effects if that's alright. Thanks!

[Carreau]

Completely forgot to merge, sorry. Merging.