Add _add_childrens_day(...) to InternationalHolidays

[PPsyrius]

Proposed change

This PR adds _add_childrens_day(...) to InternationalHolidays, currently used by Romania, Moldova, and upcoming Cambodia (#1294) holiday support.

The new InternationalHolidays is defaulted to June 1 date, though another popular variation, November 20th is also supported via self._add_childrens_day(name, NOV)

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency upgrade (version update)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new python-holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• This PR is filed against beta branch of the repository
• This PR doesn't contain any merge conflicts and has clean commit history
• The code style looks good: make pre-commit
• All tests pass locally: make test, make tox (we strongly encourage adding tests to your code)
• The related documentation has been added/updated (check off the box for free if no updates is required)

[coveralls]

coverage: 100.0%. remained the same when pulling 52177be on PPsyrius:international_childrens_day into bf54ecd on dr-prodigy:beta.

[PPsyrius]

The suggestion is now implemented for wider support, though the coverall test seems dead for some reason, even if the local tox test works perfectly.

[arkid15r]

LGTM

[KJhellico]

It has been merged into beta. Now you can use it for Cambodia holidays ;)