Implementation of toZonedDateTimeISO for Instant (#258)

Fixes #246 

Implementation of toZonedDateTimeISO for Instant.

most tests pass, one strange test262 function looks like it should pass,
but doesn't somehow.


https://github.com/tc39/test262/blob/main/test/built-ins/Temporal/Instant/prototype/toZonedDateTimeISO/timezone-string-datetime.js

This one in particular.. The code does indeed throw a range error when
trying to create a timezone from those strings.. but the test doesn't
pass.

Author: lockels

src/builtins/core/instant.rs

@@ -17,7 +17,7 @@ use crate::{
     provider::TimeZoneProvider,
     rounding::{IncrementRounder, Round},
     time::EpochNanoseconds,
-    TemporalError, TemporalResult, TemporalUnwrap, TimeZone,
+    Calendar, TemporalError, TemporalResult, TemporalUnwrap, TimeZone,
 };
 
 use ixdtf::parsers::records::UtcOffsetRecordOrZ;
@@ -236,8 +236,8 @@ impl Instant {
     }
 
     // TODO: May end up needing a provider API during impl
-    pub fn to_zoned_date_time_iso(&self, _time_zone: TimeZone) -> TemporalResult<ZonedDateTime> {
-        Err(TemporalError::general("Not yet implemented"))
+    pub fn to_zoned_date_time_iso(&self, time_zone: TimeZone) -> ZonedDateTime {
+        ZonedDateTime::new_unchecked(*self, Calendar::default(), time_zone)
     }
 }