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.cpp

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Date.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Temporal/PlainDate.h>
+#include <LibJS/Runtime/Temporal/PlainDateTime.h>
+#include <LibJS/Runtime/Temporal/PlainTime.h>
+
+namespace JS::Temporal {
+
+// 5.5.1 GetEpochFromISOParts ( year, month, day, hour, minute, second, millisecond, microsecond, nanosecond ), https://tc39.es/proposal-temporal/#sec-temporal-getepochfromisoparts
+BigInt* get_epoch_from_iso_parts(GlobalObject& global_object, i32 year, i32 month, i32 day, i32 hour, i32 minute, i32 second, i32 millisecond, i32 microsecond, i32 nanosecond)
+{
+    auto& vm = global_object.vm();
+
+    // 1. Assert: year, month, day, hour, minute, second, millisecond, microsecond, and nanosecond are integers.
+
+    // 2. Assert: ! IsValidISODate(year, month, day) is true.
+    VERIFY(is_valid_iso_date(year, month, day));
+
+    // 3. Assert: ! IsValidTime(hour, minute, second, millisecond, microsecond, nanosecond) is true.
+    VERIFY(is_valid_time(hour, minute, second, millisecond, microsecond, nanosecond));
+
+    // 4. Let date be ! MakeDay(𝔽(year), 𝔽(month − 1), 𝔽(day)).
+    auto date = make_day(global_object, Value(year), Value(month - 1), Value(day));
+
+    // 5. Let time be ! MakeTime(𝔽(hour), 𝔽(minute), 𝔽(second), 𝔽(millisecond)).
+    auto time = make_time(global_object, Value(hour), Value(minute), Value(second), Value(millisecond));
+
+    // 6. Let ms be ! MakeDate(date, time).
+    auto ms = make_date(date, time);
+
+    // 7. Assert: ms is finite.
+    VERIFY(ms.is_finite_number());
+
+    // 8. Return ℝ(ms) × 10^6 + microsecond × 10^3 + nanosecond.
+    return js_bigint(vm.heap(), Crypto::SignedBigInteger::create_from(static_cast<i64>(ms.as_double())).multiplied_by(Crypto::UnsignedBigInteger { 1'000'000 }).plus(Crypto::SignedBigInteger::create_from((i64)microsecond * 1000)).plus(Crypto::SignedBigInteger(nanosecond)));
+}
+
+}