LibJS: Add an Iterator constructor and object

The Iterator object cannot be constructed directly but can be subclassed
or created with `Iterator.from` (not implemented here).

Userland/Libraries/LibJS/Runtime/IteratorConstructor.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Forward.h>
+#include <LibJS/Runtime/NativeFunction.h>
+
+namespace JS {
+
+class IteratorConstructor : public NativeFunction {
+    JS_OBJECT(IteratorConstructor, NativeFunction);
+
+public:
+    virtual ThrowCompletionOr<void> initialize(Realm&) override;
+
+    virtual ThrowCompletionOr<Value> call() override;
+    virtual ThrowCompletionOr<NonnullGCPtr<Object>> construct(FunctionObject& new_target) override;
+
+private:
+    explicit IteratorConstructor(Realm&);
+
+    virtual bool has_constructor() const override { return true; }
+};
+
+}