Positioning responsive props do not support negative values indicated in docs

[thompsongl]

Unsure if the documentation is incorrect or if some functionality is missing, but negative spacing values have no corresponding CSS. For instance, TypeScript allows <Box top="-1" /> which applies a class of -rt-r-top-1 but no style definition for that class exists.

Seems straightforward enough to add the missing class definitions if the intent is to allow these values. I'd be happy to open a PR if that's the case.