LibJS: Add the WeakSet built-in object

Userland/Libraries/LibJS/Runtime/WeakSet.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashTable.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class WeakSet : public Object {
+    JS_OBJECT(WeakSet, Object);
+
+public:
+    static WeakSet* create(GlobalObject&);
+
+    explicit WeakSet(Object& prototype);
+    virtual ~WeakSet() override;
+
+    HashTable<Object*> const& values() const { return m_values; };
+    HashTable<Object*>& values() { return m_values; };
+
+private:
+    HashTable<Object*> m_values;
+};
+
+}