fix(lib): use optional instead of mutable defaults

[jeertmans]

No description provided.

[codecov]

Codecov Report

Attention: 45 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
manim_slides/__init__.py	60.00% <100.00%> (ø)
manim_slides/__main__.py	71.42% <100.00%> (-0.67%)	⬇️
manim_slides/commons.py	96.15% <ø> (ø)
manim_slides/config.py	84.13% <100.00%> (+0.22%)	⬆️
manim_slides/logger.py	100.00% <100.00%> (ø)
manim_slides/present/player.py	76.62% <100.00%> (ø)
manim_slides/slide/manimlib.py	100.00% <100.00%> (ø)
manim_slides/slide/animation.py	97.56% <97.56%> (ø)
manim_slides/utils.py	89.74% <83.33%> (-2.15%)	⬇️
manim_slides/convert.py	87.60% <70.00%> (+13.41%)	⬆️
... and 6 more

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