Expose a TS Generic for typing util functions

[peduarte]

Curently, if I want to type a util function against a theme scale, I need to do this:

fs: () => (value: keyof typeof theme["fontSizes"] | (string & {})) => ({
      fontSize: value
}),

Would be nice to facilitate the DX here. Perhaps something like:

fs: () => (value: Theme<"fontSizes">) => ({
      fontSize: value
}),

Or something like that 🙂

[peduarte]

@hadihallak as part of #118 can we have a think about this one too?