Add num_days_in_month method to Datelike trait

[aslilac]

The recent num_days addition to the Month type is great, but it could be even better if we took it just a little bit further.

This change adds a num_days_in_month function to the Datelike trait.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.08%. Comparing base (fa957cc) to head (97ee174).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1673      +/-   ##
==========================================
+ Coverage   91.06%   91.08%   +0.01%     
==========================================
  Files          37       37              
  Lines       17454    17469      +15     
==========================================
+ Hits        15895    15912      +17     
+ Misses       1559     1557       -2     

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[djc]

Please squash your commits into clean history:

• One for the in_leap_year() addition
• One for adding num_days_in_month()

[djc]

Thanks!