Faster xenium polygon parsing via ragged arrays

[LucaMarconato]

Parsing polygons for Xenium is now ~9x faster thanks to using from_ragged_array() and using plain numpy instead of pandas group_by.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.08%. Comparing base (1431911) to head (dc8e297).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/xenium.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
+ Coverage   62.98%   63.08%   +0.10%     
==========================================
  Files          27       27              
  Lines        3147     3153       +6     
==========================================
+ Hits         1982     1989       +7     
+ Misses       1165     1164       -1     

Files with missing lines	Coverage Δ
src/spatialdata_io/__main__.py	87.22% <ø> (-0.08%)	⬇️
src/spatialdata_io/readers/xenium.py	73.27% <89.47%> (+0.84%)	⬆️

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