refactor cells toc hiding methodology

[disberd]

This PR change the internal logic for hiding cells in the notebook based on the ToC rows hiding status is now relying purely on JS attributes attached to cells rather than complex dynamic CSS, simplifying logic and allowing to fix #46 .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.98%. Comparing base (7f609b5) to head (d0eea5c).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   98.98%   98.98%           
=======================================
  Files          16       16           
  Lines         791      791           
=======================================
  Hits          783      783           
  Misses          8        8           

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