LibJS: Implement Iterator.prototype.map

This uses a new Iterator type called IteratorHelper. This does not
implement IteratorHelper.prototype.return as that relies on generator
objects (i.e. the internal slots of JS::GeneratorObject), which are not
hooked up here.

Userland/Libraries/LibJS/Runtime/IteratorHelper.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Completion.h>
+#include <LibJS/Runtime/Iterator.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/SafeFunction.h>
+
+namespace JS {
+
+class IteratorHelper final : public Object {
+    JS_OBJECT(IteratorHelper, Object);
+
+public:
+    using Closure = JS::SafeFunction<ThrowCompletionOr<Value>(IteratorHelper&)>;
+
+    static ThrowCompletionOr<NonnullGCPtr<IteratorHelper>> create(Realm&, IteratorRecord, Closure);
+
+    IteratorRecord const& underlying_iterator() const { return m_underlying_iterator; }
+    Closure& closure() { return m_closure; }
+
+    size_t counter() const { return m_counter; }
+    void increment_counter() { ++m_counter; }
+
+    Value result(Value);
+    ThrowCompletionOr<Value> close_result(Completion);
+
+    bool done() const { return m_done; }
+
+private:
+    IteratorHelper(Object& prototype, IteratorRecord, Closure);
+
+    virtual void visit_edges(Visitor&) override;
+
+    IteratorRecord m_underlying_iterator; // [[UnderlyingIterator]]
+    Closure m_closure;
+
+    size_t m_counter { 0 };
+    bool m_done { false };
+};
+
+}