LibJS: Add the FinalizationRegistry built-in object

As well as the needed functionality in VM to enqueue and run cleanup
jobs for the FinalizationRegistry instances.

Userland/Libraries/LibJS/Runtime/FinalizationRegistry.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/SinglyLinkedList.h>
+#include <LibJS/Runtime/Function.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Value.h>
+#include <LibJS/Runtime/WeakContainer.h>
+
+namespace JS {
+
+class FinalizationRegistry final
+    : public Object
+    , public WeakContainer {
+    JS_OBJECT(FinalizationRegistry, Object);
+
+public:
+    static FinalizationRegistry* create(GlobalObject&, Function&);
+
+    explicit FinalizationRegistry(Object& prototype, Function&);
+    virtual ~FinalizationRegistry() override;
+
+    void add_finalization_record(Cell& target, Value held_value, Object* unregister_token);
+    bool remove_by_token(Object& unregister_token);
+    void cleanup(Function* callback = nullptr);
+
+    virtual void remove_sweeped_cells(Badge<Heap>, Vector<Cell*>&) override;
+
+private:
+    virtual void visit_edges(Visitor& visitor) override;
+
+    Function* m_cleanup_callback { nullptr };
+
+    struct FinalizationRecord {
+        Cell* target { nullptr };
+        Value held_value;
+        Object* unregister_token { nullptr };
+    };
+    SinglyLinkedList<FinalizationRecord> m_records;
+};
+
+}