Extend ISO 8601 support to include years BCE and > 9999 CE.

A 2019 update to ISO 8601 extended it to allow years outside the range from 0000 (1 BCE) to 9999; the year number, if it's not simply four digits, must start with a + or - sign and give all of its digits. Formally this only applies where both parties to the information exchange have agreed to it, and agreed the number of digits to use.
It may thus make sense to introduce a third ISODate member to the time format enum, or to change how we handle ISO date formatting entirely, to an API with optional flags (for inclusion of milliseconds, extended years) and parameters (number of digits to give in extended years).
However, on parsing, it makes sense to simply be liberal; and it may be adequate to simply stick with the enum members we have and handle formerly-out-of-range years with a sign and its minimal number of digits.