Cross-Calendar issues

[steida]

https://tc39.es/proposal-temporal/docs/calendar.html#writing-cross-calendar-code

• [ ] When looping through all months in a year, use monthsInYear as the upper bound instead of assuming that every year has 12 months.
• [ ] Days in a month are not always continuous. There can be gaps due to political changes in calendars and/or time zones. For this reason, instead of looping through a month from 1 to date.daysInMonth, it's better to start a loop with the first day of the month (.with({day: 1})) and add one day at a time until the month property returns a different value.

Is that all?

[steida]

I removed the bug label because calendar: "iso8601" works—this issue is for supporting other calendars.