fix(lib): rewrite FSM and patch multiple presentation bugs

[jeertmans]

I small rewrite of the Finite State Machine was needed to fix a lot of issues when transitioning from one presentation to another.

Here are a few:

• loading last slide from previous presentation would actually load the second last;
• if the first animation of some presentation was played in reverse, then going to the previous presentation, then back to the current, would play the first slide in reverse again
• no pause was performed between two presentation, this is now automatic

[codecov]

Codecov Report

Patch coverage: 27.27% and project coverage change: +0.24% 🎉

Comparison is base (9e1e0f2) 59.11% compared to head (e1ac428) 59.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
+ Coverage   59.11%   59.35%   +0.24%     
==========================================
  Files          14       14              
  Lines        1734     1737       +3     
==========================================
+ Hits         1025     1031       +6     
+ Misses        709      706       -3     

Files Changed	Coverage Δ
manim_slides/present.py	57.35% <27.27%> (+0.71%)	⬆️

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