Previously getISOFields() was used to get the exact value of the
[[Calendar]] and [[TimeZone]] internal slots, as well as to get the
reference ISO year for PlainMonthDay and reference ISO day for
PlainYearMonth.
Use calendarId and timeZoneId for the former and toString() for the
latter.
The idea is to deduplicate more string tests into methods on this object,
that return collections of valid and invalid strings. This adds
collections of valid and invalid PlainYearMonth and PlainMonthDay strings.