LibJS: Implement Temporal.PlainMonthDay.prototype.toString()

Author: https://github.com/linusg
Commit: ea44f33d5b
Pull-request: https://github.com/SerenityOS/serenity/pull/9510
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDay.h

@@ -40,5 +40,6 @@ struct ISOMonthDay {
 };
 
 PlainMonthDay* create_temporal_month_day(GlobalObject&, u8 iso_month, u8 iso_day, Object& calendar, i32 reference_iso_year, FunctionObject* new_target = nullptr);
+Optional<String> temporal_month_day_to_string(GlobalObject&, PlainMonthDay&, StringView show_calendar);
 
 }