Don't build the JupyterLab extension by default

[mwouts]

Closes #1210

[github-actions]

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1233   +/-   ##
=======================================
  Coverage   97.75%   97.75%           
=======================================
  Files          29       29           
  Lines        4464     4464           
=======================================
  Hits         4364     4364           
  Misses        100      100           

Flag	Coverage Δ
external	75.17% <ø> (ø)
functional	88.51% <ø> (ø)
integration	77.28% <ø> (ø)
unit	66.56% <ø> (ø)

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