Add support for #undef

[Schamper]

As the title implies.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.98%. Comparing base (bbbb86e) to head (c6b8ff4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   92.95%   92.98%   +0.02%     
==========================================
  Files          21       21              
  Lines        2427     2437      +10     
==========================================
+ Hits         2256     2266      +10     
  Misses        171      171              

Flag	Coverage Δ
unittests	92.98% <100.00%> (+0.02%)	⬆️

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