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/PlainYearMonth.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Temporal/PlainYearMonth.h>
+
+namespace JS::Temporal {
+
+// 9.5.5 BalanceISOYearMonth ( year, month ), https://tc39.es/proposal-temporal/#sec-temporal-balanceisoyearmonth
+ISOYearMonth balance_iso_year_month(i32 year, i32 month)
+{
+    // 1. Assert: year and month are integers.
+
+    // 2. Set year to year + floor((month - 1) / 12).
+    year += (month - 1) / 12;
+
+    // 3. Set month to (month − 1) modulo 12 + 1.
+    month = (month - 1) % 12 + 1;
+
+    // 4. Return the new Record { [[Year]]: year, [[Month]]: month }.
+    return ISOYearMonth { .year = year, .month = static_cast<u8>(month) };
+}
+
+}