Update theme.fontSize types

[bradlc]

This PR updates the theme.fontSize types so so you can mix the different formats. Currently all of the values need to have the same format, e.g.

// ✅
module.exports = {
  theme: {
    fontSize: {
      foo: '4rem',
      bar: '5rem',
    },
  },
}

// ✅
module.exports = {
  theme: {
    fontSize: {
      foo: ['4rem', '6rem'],
      bar: ['5rem', '7rem'],
    },
  },
}

// ❌
module.exports = {
  theme: {
    fontSize: {
      foo: '4rem',
      bar: ['5rem', '7rem'],
    },
  },
}

[RobinMalfait]

Hah, I totally forgot that you could mix those values and that you don't have to choose between one syntax or another for everything. This makes sense, thanks!