LibJS: Implement Temporal.Now.plainDate()

...and ten required AOs we didn't have yet:

- BalanceISODate
- BalanceISODateTime
- BalanceISOYearMonth
- BalanceTime
- BuiltinTimeZoneGetPlainDateTimeFor
- GetISOPartsFromEpoch
- GetOffsetNanosecondsFor
- ParseTemporalTimeZone
- SystemDateTime
- ToTemporalTimeZone

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

@@ -92,6 +92,25 @@ bool iso_date_time_within_limits(GlobalObject& global_object, i32 year, u8 month
     return true;
 }
 
+// 5.5.5 BalanceISODateTime ( year, month, day, hour, minute, second, millisecond, microsecond, nanosecond ), https://tc39.es/proposal-temporal/#sec-temporal-balanceisodatetime
+ISODateTime balance_iso_date_time(i32 year, u8 month, u8 day, u8 hour, u8 minute, u8 second, u16 millisecond, u16 microsecond, i64 nanosecond)
+{
+    // NOTE: The only use of this AO is in BuiltinTimeZoneGetPlainDateTimeFor, where we know that all values
+    // but `nanosecond` are in their usual range, hence why that's the only outlier here. The range for that
+    // is -86400000000000 to 86400000000999, so an i32 is not enough.
+
+    // 1. Assert: year, month, day, hour, minute, second, millisecond, microsecond, and nanosecond are integers.
+
+    // 2. Let balancedTime be ! BalanceTime(hour, minute, second, millisecond, microsecond, nanosecond).
+    auto balanced_time = balance_time(hour, minute, second, millisecond, microsecond, nanosecond);
+
+    // 3. Let balancedDate be ! BalanceISODate(year, month, day + balancedTime.[[Days]]).
+    auto balanced_date = balance_iso_date(year, month, day + balanced_time.days);
+
+    // 4. Return the Record { [[Year]]: balancedDate.[[Year]], [[Month]]: balancedDate.[[Month]], [[Day]]: balancedDate.[[Day]], [[Hour]]: balancedTime.[[Hour]], [[Minute]]: balancedTime.[[Minute]], [[Second]]: balancedTime.[[Second]], [[Millisecond]]: balancedTime.[[Millisecond]], [[Microsecond]]: balancedTime.[[Microsecond]], [[Nanosecond]]: balancedTime.[[Nanosecond]] }.
+    return ISODateTime { .year = balanced_date.year, .month = balanced_date.month, .day = balanced_date.day, .hour = balanced_time.hour, .minute = balanced_time.minute, .second = balanced_time.second, .millisecond = balanced_time.millisecond, .microsecond = balanced_time.microsecond, .nanosecond = balanced_time.nanosecond };
+}
+
 // 5.5.6 CreateTemporalDateTime ( isoYear, isoMonth, isoDay, hour, minute, second, millisecond, microsecond, nanosecond, calendar [ , newTarget ] ), https://tc39.es/proposal-temporal/#sec-temporal-createtemporaldatetime
 PlainDateTime* create_temporal_date_time(GlobalObject& global_object, i32 iso_year, u8 iso_month, u8 iso_day, u8 hour, u8 minute, u8 second, u16 millisecond, u16 microsecond, u16 nanosecond, Object& calendar, FunctionObject* new_target)
 {