Add YearMonth, MonthDay

[ariebovenberg]

Description

These types are common in other modern datetime libraries. They're straightforward to add.

Checklist

• Build runs successfully
• Type stubs updated
• Docs updated
• If docstrings were affected, check if they appear correctly in the docs as well as autocomplete

Release checklist (maintainers only)

• Version updated in pyproject.toml
• Version updated in src/whenever/_pywhenever.py
• Version updated in changelog
• Branch merged
• Tag created and pushed
• Confirm published job runs successfully
• Github release created