Rename roman to not-italic

[adamwathan]

I made this same change way back in v0.5.0 but had to revert it due to a bug in postcss-selector-not that made it impossible for people to use Tailwind with that plugin becuase of classes like sm:not-italic being interpreted as containing a :not pseudo-selector.

That has since been fixed, and postcss-preset-env (the library most people would be transitively depending on postcss-selector-not from) has been tagged to use the fixed version, so I think (especially for 1.0) that it is safe to finally make this change.

[missmatsuko]

I haven't actually used oblique before, but font-style can be normal, italic, or oblique (with an optional angle). Is it weird to name it not-italic when it's also not-oblique?

[adamwathan]

It is a little weird but we have the same issue with underline, no-underline, and line-through, where line-through affects the same property but you need to use no-underline to undo it. I can live with it, it's not the sort of thing you tweak that much responsively anyways.