fix ExtendedTableOfContents

[disberd]

This PR fixes a bug breaking the functionality of ExtendedTableOfContents with recent versions of PlutoUI.

This commit JuliaPluto/PlutoUI.jl@4fb2d5a (or someone close to it) changed some internals that broke how PlutoExtras was handling the ToC to add the extended functionality.

This PR makes the code handling the processing more robust and also refactors the code so that it's separated into a .js file (containing most of the scripts content), a .css file containing the style and a much reduced julia file which simply interpolated the JS and CSS code directly inside the @htl("...") macro, leading to cleaner code that is much easier to reason about (especially thanks to proper code formatting in IDEs for the JS and CSS part).

As part of this PR, I also fixed an issue with the hide heading button that was going out of position when scrolling the notebook.

I also tested again on firefox and I can't experince the issue in #28 in firefox, so this also fixes #28

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.94%. Comparing base (8575360) to head (5dd5853).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #45   +/-   ##
=======================================
  Coverage   98.94%   98.94%           
=======================================
  Files          16       16           
  Lines         758      758           
=======================================
  Hits          750      750           
  Misses          8        8           

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