Add Month::num_days()

[djc]

Suggested in #1643.

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 10 lines in your changes missing coverage. Please review.

Project coverage is 90.87%. Comparing base (8b86349) to head (afb9299).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/month.rs	54.54%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1645      +/-   ##
==========================================
- Coverage   91.11%   90.87%   -0.24%     
==========================================
  Files          37       37              
  Lines       17137    17143       +6     
==========================================
- Hits        15614    15579      -35     
- Misses       1523     1564      +41     

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