Add Japan calendar support (issue #142)

[yangminqiu]

Summary

• add Market::Japan to calendar enum and holiday dispatch
• add countries/japan.rs with modern Japanese public holiday rules
• include unit tests for 2025 holidays + sample business days

Notes

• Implements core national holiday set, happy-Monday holidays, equinox approximations, and Sunday substitute Mondays.
• This addresses one of the unchecked countries in Add more calendars.  #142.

Refs #142