LibJS: Generify the garbage collector's weak container notifications

This will allow us to use the same interface for other JS weak
containers like the WeakMap & WeakRef.

Userland/Libraries/LibJS/Runtime/WeakContainer.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Heap/Heap.h>
+
+namespace JS {
+
+class WeakContainer {
+public:
+    explicit WeakContainer(Heap& heap)
+        : m_heap(heap)
+    {
+        m_heap.did_create_weak_container({}, *this);
+    }
+    virtual ~WeakContainer()
+    {
+        m_heap.did_destroy_weak_container({}, *this);
+    }
+
+    virtual void remove_sweeped_cells(Badge<Heap>, Vector<Cell*>&) = 0;
+
+private:
+    Heap& m_heap;
+};
+
+}