Calendar precedence in DateTimeFormat
Calendar precedence in DateTimeFormat

For Intl.DateTimeFormat and toLocaleString(), the Temporal object's
calendar should take precedence if it is non-ISO (which here we define
as not 'iso8601' or 'gregory'), and otherwise the locale's calendar
should take precedence.

For ranges, we throw if two Temporal objects are given with differing
calendars that are both non-ISO.

See: #262

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill