Add `CalendarDuration` type

[punkstarman]

picking up from #1290

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.15385% with 36 lines in your changes missing coverage. Please review.

Project coverage is 90.54%. Comparing base (d1de1d9) to head (3d16fcd). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/calendar_duration.rs	86.15%	36 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1699      +/-   ##
==========================================
- Coverage   90.61%   90.54%   -0.07%     
==========================================
  Files          38       39       +1     
  Lines       16945    17205     +260     
==========================================
+ Hits        15354    15578     +224     
- Misses       1591     1627      +36     

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