Add setdiff! for Polygon and TriangleMesh

[LasNikas]

The example can now produce something like this

[codecov]

Codecov Report

Attention: Patch coverage is 16.98113% with 44 lines in your changes missing coverage. Please review.

Project coverage is 68.83%. Comparing base (6f3894c) to head (768d1d7).

Files with missing lines	Patch %	Lines
src/preprocessing/geometries/polygon.jl	20.45%	35 Missing ⚠️
src/preprocessing/geometries/io.jl	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #633      +/-   ##
==========================================
- Coverage   69.37%   68.83%   -0.55%     
==========================================
  Files          85       85              
  Lines        5124     5169      +45     
==========================================
+ Hits         3555     3558       +3     
- Misses       1569     1611      +42     

Flag	Coverage Δ
unit	68.83% <16.98%> (-0.55%)	⬇️

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