Extend letter-spacing scale

[adamwathan]

This PR extends the default letter-spacing scale to include the following values:

{
  // ...
  letterSpacing: {
    tighter: '-.05em',
    tight: '-.025em',
    normal: '0',
    wide: '.025em',
    wider: '.05em',
    widest: '.1em',
  },
}

I find I am almost always changing the default tight and wide from .05em to .025em, so I've changed those here but kept the .05em values as tighter and wider. I've also added a widest option at 0.1em, but chose not to mirror that to tightest because it would be far too tight to be useful.

This is a breaking change for anyone who was inheriting our default values in their project but easily solved by locking in the old values in your own config.

[sandren]

The values for tight and wide may change slightly, but I imagine many will just accept the new defaults and the accompanying design changes.

I like the new tracking scale, but would add that type extended beyond the widest class is a pretty common design pattern. You frequently see small headers or labels use type that is bold and uppercase with very extended tracking. Therefore, I think it would be helpful to add .tracking-extended { .15em; } so that Tailwind covers most everyday use cases.

[adamwathan]

Going to stick with just these 6 for now but can always add a new extended value in a future release since it wouldn't be a breaking change 👍 Easy to extend the scale per project in the mean time.