feat: add support for single-color colormaps with constant mapping behavior

[tlambert03]

closes #139

hey @aafkegros, here's one possible fix to #139. Let me know how this would work for you. And again, still happy to iterate on this

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.56%. Comparing base (954b1ca) to head (eedc86c).

Files with missing lines	Patch %	Lines
src/cmap/_colormap.py	83.33%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (83.33%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
- Coverage   95.56%   93.56%   -2.00%     
==========================================
  Files         168      168              
  Lines        2186     2191       +5     
==========================================
- Hits         2089     2050      -39     
- Misses         97      141      +44     

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