LibJS: Add Add IsValidISODate assertions

This is an editorial change in the Temporal spec.

See: 46f97ea

Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp

@@ -155,6 +155,7 @@ MarkedVector<BigInt*> get_iana_time_zone_epoch_value(GlobalObject& global_object
     // The implementation-defined abstract operation GetIANATimeZoneEpochValue takes arguments timeZoneIdentifier (a String), year (an integer), month (an integer between 1 and 12 inclusive), day (an integer between 1 and 31 inclusive), hour (an integer between 0 and 23 inclusive), minute (an integer between 0 and 59 inclusive), second (an integer between 0 and 59 inclusive), millisecond (an integer between 0 and 999 inclusive), microsecond (an integer between 0 and 999 inclusive), and nanosecond (an integer between 0 and 999 inclusive) and returns a List of BigInts.
     // Each value in the returned List represents a number of nanoseconds since the Unix epoch in UTC that corresponds to the given ISO 8601 calendar date and wall-clock time in the IANA time zone identified by timeZoneIdentifier.
     // When the input represents a local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change), the returned List will have more than one element. The elements are sorted in numerical order. When the input represents a skipped local time at a positive time zone transition (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change), the returned List will be empty. Otherwise, the returned List will have one element.
+    // It is an error to call GetIANATimeZoneEpochValue with arguments such that IsValidISODate(year, month, day) is false.
 
     // FIXME: Implement this properly for non-UTC timezones.
     auto& vm = global_object.vm();