`styles.order` should be on `FLEX_ITEM`

[chrislopresto]

Hey folks. Thanks for the fine work and for open-sourcing it as MIT.

When perusing your source for inspiration for our design system, I noticed that styles.order was included on the FLEX_CONTAINER constant rather than the FLEX_ITEM constant.

https://github.com/primer/components/blob/204f45402fad9d8d1a3e6525fbf9881bc9c51980/src/constants.js#L50-L65

It probably belongs on FLEX_ITEM, right? I figured I'd open an issue to a) mention it in case you weren't aware and b) send some drive-by thanks.

Thanks! 🎉

[emplums]

Oh hey nice catch @chrislopresto! I'll fix that tomorrow :) Thanks for the heads up!