LibJS: Implement Set.prototype.union

Userland/Libraries/LibJS/Runtime/Set.cpp

@@ -23,6 +23,18 @@ void Set::initialize(Realm& realm)
     m_values = Map::create(realm);
 }
 
+NonnullGCPtr<Set> Set::copy() const
+{
+    auto& vm = this->vm();
+    auto& realm = *vm.current_realm();
+    // FIXME: This is very inefficient, but there's no better way to do this at the moment, as the underlying Map
+    //  implementation of m_values uses a non-copyable RedBlackTree.
+    auto* result = Set::create(realm);
+    for (auto const& entry : *this)
+        result->set_add(entry.key);
+    return *result;
+}
+
 void Set::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);