feat(convert): allow fully offline HTML presentation

[jeertmans]

TODO: check if this is really the case, especially for nested dependencies?

Closes #438

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.05%. Comparing base (75af26e) to head (debd631). Report is 47 commits behind head on main.

Files with missing lines	Patch %	Lines
manim_slides/convert.py	96.42%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #440      +/-   ##
==========================================
+ Coverage   80.86%   81.05%   +0.18%     
==========================================
  Files          23       23              
  Lines        1866     1884      +18     
==========================================
+ Hits         1509     1527      +18     
  Misses        357      357              

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