Undocumented scroll event argument positionX

I stumbled onto the second argument passed to the scroll event, `positionX`, and am wondering if it's reliable to use it. It's not in the documentation, but it looks like it's been there since v2.0.0.
Can I use this in production and not worry about it being removed in the future?

Here's the line where the scroll event gets returned with 2 arguments: `progress, positionX` https://github.com/metafizzy/flickity/blob/e2706840532c0ce9c4fc25832e810ad4f9823b61/js/animate.js#L94

Thanks!

Start with the scroll event return in js/animate.js at line 94 and inspect the surrounding event implementation. Determine whether positionX is a supported public argument, then document its status and stability in the relevant API documentation; done means users can tell whether it is safe to rely on.

Written by the indexing model from the issue text.