Responsive object style about system-props

[roginfarrer]

Thanks for the report, this is one of the smaller edge cases when I developed types that I wasn't sure how to resolve. Currently the types accept any breakpoint theme value or 'all'

https://github.com/system-props/system-props/blob/main/system-props/src/types/system-props.ts#L228-230

Curious what you think the solution should be here. The types could expand to accept other common aliases for this, like '_' and 'initial'. I don't think we want to accept any value (though in theory that's true, any non-breakpoint value would be the same as 'all', but I don't think that's a good UX)