Implement missing plot functionality for pl.rank_genes_groups

[flying-sheep]

• Closes Rank_genes_groups -> does not return Axes #3205
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.65%. Comparing base (72b0b81) to head (28d18ab).

Files with missing lines	Patch %	Lines
src/scanpy/plotting/_tools/__init__.py	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3428      +/-   ##
==========================================
- Coverage   76.67%   76.65%   -0.02%     
==========================================
  Files         112      112              
  Lines       12945    12955      +10     
==========================================
+ Hits         9925     9931       +6     
- Misses       3020     3024       +4     

Files with missing lines	Coverage Δ
src/scanpy/plotting/_tools/__init__.py	77.74% <77.77%> (-0.48%)	⬇️