Support sx/css prop

[nandorojo]

Rather than have just direct style props for components, theme-ui has an sx object prop. I find it to be a really nice design pattern for styles. However, it is a bit more challenging to memoize.

I understand if this isn't the philosophy of this library, and maybe it isn't something you're interested in implementing. But I find it to be a really nice way to decouple styles from other props.

Either way, great work with the library! Nice to see a design system with React Native as a first-class citizen.