AK+LibJS: Introduce JS::HeapFunction

This change introduces HeapFunction, which is intended to be used as a
replacement for SafeFunction. The new type behaves like a regular
GC-allocated object, which means it needs to be visited from
visit_edges, and unlike SafeFunction, it does not create new roots for
captured parameters.

Co-Authored-By: Andreas Kling <kling@serenityos.org>

Userland/Libraries/LibJS/Heap/HeapFunction.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <LibJS/Heap/Cell.h>
+#include <LibJS/Heap/Heap.h>
+
+namespace JS {
+
+template<typename T>
+class HeapFunction final : public JS::Cell {
+    JS_CELL(HeapFunction, Cell);
+
+public:
+    static NonnullGCPtr<HeapFunction> create(Heap& heap, Function<T> function)
+    {
+        return heap.allocate_without_realm<HeapFunction>(move(function));
+    }
+
+    virtual ~HeapFunction() override = default;
+
+    Function<T> const& function() const { return m_function; }
+
+private:
+    HeapFunction(Function<T> function)
+        : m_function(move(function))
+    {
+    }
+
+    virtual void visit_edges(Visitor& visitor) override
+    {
+        Base::visit_edges(visitor);
+        visitor.visit_possible_values(m_function.raw_capture_range());
+    }
+
+    Function<T> m_function;
+};
+
+template<typename T>
+static NonnullGCPtr<HeapFunction<T>> create_heap_function(Heap& heap, Function<T> function)
+{
+    return HeapFunction<T>::create(heap, move(function));
+}
+
+}