Shorthand descending sort

It’d be nice to have a shorthand for descending sort such as:

```js
Plot.barY(alphabet, {x: "letter", y: "frequency", sort: {x: "-y"}}).plot()
```

This would be equivalent to:

```js
Plot.barY(alphabet, {x: "letter", y: "frequency", sort: {x: "y", reverse: true}}).plot()
```