Allow for nested themes

[joseDaKing]

Allow for nested themes, It would allow for logical grouping and it should not be used with theme mapping.

[joseDaKing]

It would be nice if it is supported but it is unnecessary because I can group the team vars manually

const { theme: vars } = createStitches(...);

const theme = {
  color: {
    primary: {
      1: theme["primary-1"],
      2: theme["primary-2"],
      3: theme["primary-3"],
      ....
    }
    ....
  },
  ...
};