The first step of to_temporal_calendar_with_iso_default() is checking
whether the given value is undefined, so we should actually pass that
instead of unconditionally dereferencing the Optional<String>.
I changed this in 6ef1a27 to "match the spec", but the spec calls it
`match exactly` and `match minutes` - so what we had before was correct
and the change made no sense whatsoever.
There's no need for these to be non-const. Suggested by @IdanHo in
https://github.com/SerenityOS/serenity/pull/9904#discussion_r704960184.
Perhaps we can make more internal slots of these and other objects const
references as well, but that's a bit more involved as they are used by
various functions expecting non-const references.
This commit adds the ZonedDateTime object itself, its constructor and
prototype (currently empty), and the CreateTemporalZonedDateTime
abstract operation.
