MERSCOPE: adding feature_key attribute for points

[quentinblampey]

Minor (but useful) PR: simply passing "gene" to the feature_key argument when parsing the points.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.50%. Comparing base (54da345) to head (cff4896).
Report is 202 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
+ Coverage   35.91%   45.50%   +9.59%     
==========================================
  Files          19       22       +3     
  Lines        1715     2149     +434     
==========================================
+ Hits          616      978     +362     
- Misses       1099     1171      +72     

Files with missing lines	Coverage Δ
src/spatialdata_io/_constants/_constants.py	100.00% <100.00%> (ø)
src/spatialdata_io/readers/merscope.py	24.06% <ø> (-0.94%)	⬇️

... and 4 files with indirect coverage changes

[LucaMarconato]

Thanks! I noticed I had made a mistake in the changelog; I fixed it.