Feature request: scale_x_upset order_by weighted frequency

I've been using ggupset for datasets where each datapoint has a weight, but when you set the `weight` aesthetic in `geom_bar`, the order generated by `scale_x_upset(order_by = "freq")` is still the order of the unweighted data. It would be nice to have the option of sorting by weighted total.

Thanks for the great package!

Start by tracing how scale_x_upset(order_by = "freq") determines its ordering and how geom_bar consumes the weight aesthetic. Compare the current unweighted frequency with the weighted total, then verify that the new ordering option works on a weighted dataset and preserves existing unweighted behavior.

Written by the indexing model from the issue text.