LibJS: Mark heap cell values stored in Set instances

This makes sure they dont get garbage collected while stored in a Set.

Userland/Libraries/LibJS/Runtime/Set.h

@@ -47,6 +47,8 @@ public:
     HashTable<Value, ValueTraits>& values() { return m_values; };
 
 private:
+    virtual void visit_edges(Visitor& visitor) override;
+
     HashTable<Value, ValueTraits> m_values; // FIXME: Replace with a HashTable that maintains a linked list of insertion order for correct iteration order
 };