Add calendar parsing & conversion for French Republican calendar #136
Description
Calendar conversion is supported by convertdate, although there are multiple ways to calculate leap years: https://github.com/fitnr/convertdate/tree/master?tab=readme-ov-file#french-republican
This is a decimal-based calendar with 10-day cycles instead of weeks, so it would help to identify any assumptions we're making in the code about seven-day weeks (which holds for all currently supported calendars).
It would be great if we can find a project or dataset with French Republican dates for a use case and to check against.