Allow default letterSpacing in fontSize config
Allow default letterSpacing in fontSize config

Extend the configuration syntax to allow specifying letterSpacing
alongside lineHeight in the fontSize config.

The syntax looks like
```
module.exports = {
  theme: {
    // ...
    fontSize: {
      sm: ['14px', { lineHeight: '20px', letterSpacing: '0.01em' }],
      // ...
    }
  }
}
```

Also retains backwards compatibility with array syntax of 1.3

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