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.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Temporal/AbstractOperations.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS::Temporal {
+
+struct ISOYearMonth {
+    i32 year;
+    u8 month;
+};
+
+ISOYearMonth balance_iso_year_month(i32 year, i32 month);
+
+}