Remove ZonedDateTime from Formattable

[ykiu]

Closes #78

[arshaw]

This was fixed in the v1 launch, which more-or-less ported over the official types from the TypeScript project. Here's what's present now:

export namespace Intl {
  type FormattableTemporalObject =
    | Temporal.PlainDate
    | Temporal.PlainYearMonth
    | Temporal.PlainMonthDay
    | Temporal.PlainTime
    | Temporal.PlainDateTime
    | Temporal.Instant