Support breakpoints into default props like: colors, borderRadii, spacing

[Victor R.]

Hey there, you lib is really great!
I would like to ask to implement the possibility breakpoints in default props.
Example:

export const theme = createTheme({
  breakpoints: {
    phone: 0,
    tablet: 768,
  },
  spacing: {
     xxl: {
         phone: 12,
         tablet: 20
     }
  }
});