feat: render ProblemSet as DOT

[redeboer]

Closes #28

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[codecov]

Codecov Report

Merging #86 (858eee6) into main (ae6f9c3) will increase coverage by 0.08%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   93.36%   93.44%   +0.08%     
==========================================
  Files          15       15              
  Lines        3105     3159      +54     
==========================================
+ Hits         2899     2952      +53     
- Misses        206      207       +1     

Flag	Coverage Δ
unittests	93.44% <93.33%> (+0.08%)	⬆️

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Impacted Files	Coverage Δ
src/qrules/io/_dot.py	83.12% <92.85%> (+4.18%)	⬆️
src/qrules/_implementers.py	86.20% <100.00%> (+0.49%)	⬆️
src/qrules/io/__init__.py	91.30% <100.00%> (ø)