Don't build the JupyterLab extension by default

[mwouts]

Closes #1210

[github-actions[bot]]

Thank you for making this pull request.

Did you know? You can try it on Binder: or .

Also, the version of Jupytext developed in this PR can be installed with pip:

pip install git+https://github.com/mwouts/jupytext.git@no_extension_by_default

(this requires nodejs, see more at Developing Jupytext)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.75%. Comparing base (ef4a2b7) to head (407b43e). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1233 +/- ## ======================================= Coverage 97.75% 97.75% ======================================= Files 29 29 Lines 4464 4464 ======================================= Hits 4364 4364 Misses 100 100 ``` | [Flag](https://app.codecov.io/gh/mwouts/jupytext/pull/1233/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | Coverage Δ | | |---|---|---| | [external](https://app.codecov.io/gh/mwouts/jupytext/pull/1233/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `75.17% <ø> (ø)` | | | [functional](https://app.codecov.io/gh/mwouts/jupytext/pull/1233/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `88.51% <ø> (ø)` | | | [integration](https://app.codecov.io/gh/mwouts/jupytext/pull/1233/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `77.28% <ø> (ø)` | | | [unit](https://app.codecov.io/gh/mwouts/jupytext/pull/1233/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `66.56% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts#carryforward-flags-in-the-pull-request-comment) to find out more.

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