LibJS: Start implementing Temporal.Instant

Just like the initial Temporal.TimeZone commit, this patch adds the
Instant object itself, its constructor and prototype (currently empty),
and two required abstract operations.

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

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <LibJS/Runtime/BigInt.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS::Temporal {
+
+class Instant final : public Object {
+    JS_OBJECT(Instant, Object);
+
+public:
+    explicit Instant(BigInt& nanoseconds, Object& prototype);
+    virtual ~Instant() override = default;
+
+    BigInt const& nanoseconds() const { return m_nanoseconds; }
+
+private:
+    virtual void visit_edges(Visitor&) override;
+
+    // 8.4 Properties of Temporal.Instant Instances, https://tc39.es/proposal-temporal/#sec-properties-of-temporal-instant-instances
+
+    // [[Nanoseconds]]
+    BigInt& m_nanoseconds;
+};
+
+// -86400 * 10^17
+const auto INSTANT_NANOSECONDS_MIN = Crypto::SignedBigInteger::from_base(10, "-8640000000000000000000");
+// +86400 * 10^17
+const auto INSTANT_NANOSECONDS_MAX = Crypto::SignedBigInteger::from_base(10, "8640000000000000000000");
+
+bool is_valid_epoch_nanoseconds(BigInt const& epoch_nanoseconds);
+Object* create_temporal_instant(GlobalObject&, BigInt& nanoseconds, FunctionObject* new_target = nullptr);
+
+}