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/InstantConstructor.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/NativeFunction.h>
+
+namespace JS::Temporal {
+
+class InstantConstructor final : public NativeFunction {
+    JS_OBJECT(InstantConstructor, NativeFunction);
+
+public:
+    explicit InstantConstructor(GlobalObject&);
+    virtual void initialize(GlobalObject&) override;
+    virtual ~InstantConstructor() override = default;
+
+    virtual Value call() override;
+    virtual Value construct(FunctionObject& new_target) override;
+
+private:
+    virtual bool has_constructor() const override { return true; }
+};
+
+}