Provide next and previous time zone transition given a named time zone and an epoch moment

[hsivonen]

Temporal requires the operations GetNamedTimeZoneNextTransition and GetNamedTimeZonePreviousTransition, which return the next and previous moments for the named time zone relative to the given moment when the UTC offset differs from the UTC offset on the given moment.

For Temporal compat, ICU4X should provide these operations.

[sffc]

CC @leftmostcat @nordzilla