Default letter-spacing per font-size

[ben-rogerson]

This PR adds an alternative fontSize config to specify letterSpacing released in Tailwind v1.7.0.

Tailwind docs: Providing a default letter-spacing

// config
module.exports = {
  theme: {
    extend: {
      fontSize: {
        hero: [
          '7rem',
          {
            lineHeight: '80%',
            letterSpacing: '-0.02em'
          }
        ]
      },
    }
  }
}

// usage
<div tw="text-hero text-purple-600">hero text</div>

// output
;({
  fontSize: '7rem',
  lineHeight: '80%',
  letterSpacing: '-0.02em',
})