LibJS: Add the ToTemporalInstant Abstract Operation & its requirements

This is Abstract Operation is required for the majority of
InstantConstructor's and InstantPrototype's methods.

The implementation is not entirely complete, (specifically 2 of the
underlying required abstract operations, ParseTemporalTimeZoneString
and ParseISODateTime are missing the required lexing, and as such are
TODO()-ed) but the majority of it is done.

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

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/BigInt.h>
+
+namespace JS::Temporal {
+
+BigInt* get_epoch_from_iso_parts(GlobalObject&, i32 year, i32 month, i32 day, i32 hour, i32 minute, i32 second, i32 millisecond, i32 microsecond, i32 nanosecond);
+
+}