Add role property to Flex.Item, flex to Flex

[cheshire137]

I'm wondering if the role property can be added to Flex.Item, and the flex property to Flex. I'm using Primer components in my TypeScript app and stuck because these properties are missing:

Even just having one of these properties added would un-stick me. Thank you!

cc @emplums

[emplums]

This turned out to be because the @types/styled-system dependency was missing