feat(lib): allow to insert external videos as slides

[jeertmans]

See #520

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.09%. Comparing base (a2bd1ff) to head (f0dd8b0).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
manim_slides/config.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
+ Coverage   79.96%   80.09%   +0.13%     
==========================================
  Files          23       23              
  Lines        2011     2025      +14     
==========================================
+ Hits         1608     1622      +14     
  Misses        403      403              

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