LibJS: Implement a very hackish "arguments" object

We now lazily create an "arguments" array inside functions when code
tries to access it.

This doesn't follow the spec at all but still covers a lot of the
basic uses of arguments, i.e "arguments.length" and "arguments[n]"

Libraries/LibJS/Runtime/VM.h

@@ -59,6 +59,7 @@ struct CallFrame {
     FlyString function_name;
     Value this_value;
     Vector<Value> arguments;
+    Array* arguments_object { nullptr };
     ScopeObject* scope { nullptr };
     bool is_strict_mode { false };
 };