Move to content rewriting code from zimscraperlib

[benoit74]

Code to rewrite CSS, JS and HTML has been moved from warc2zim to zimscraperlib by openzim/python-scraperlib#204

This PR adapts the scraper to use CSS rewriter from zimscraperlib (formerly warc2zim) and mutualize CSS handling code to avoid repetitions.

[codecov]

Codecov Report

Attention: Patch coverage is 13.04348% with 20 lines in your changes missing coverage. Please review.

Project coverage is 45.69%. Comparing base (797edd3) to head (2bd3c61).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/libretexts2zim/processor.py	13.04%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   50.72%   45.69%   -5.04%     
==========================================
  Files           9        8       -1     
  Lines         554      499      -55     
  Branches       63       53      -10     
==========================================
- Hits          281      228      -53     
  Misses        269      269              
+ Partials        4        2       -2     

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[rgaudin]

That's the kind of PR I like ! Where most of it is deleted file 😉

Please see the dict update suggestion before merging