Add snug and relaxed line-heights

[adamwathan]

This PR adds two new line-height values:

  lineHeight: {
    none: 1,
    tight: 1.25,
+   snug: 1.375,
    normal: 1.5,
+   relaxed: 1.625,
    loose: 2,
  },

The snug value is useful for larger text that is still laid out in a paragraph-ish style, like the "Got creative ideas? ..." text on the Framer home page:

The relaxed value is useful for smaller paragraph text, close to what sites like Medium use for long form writing:

None of the existing values have been renamed or changed so this isn't a breaking change 👍

[sandren]

In graphic design, the leading is typically set to something like 1.25 for headlines, 1.5 for body copy, and captions somewhere in between. Then lists of course have something closer to 2.0 leading.

If this pull request is merged, then Tailwind would by default cover all of the approximate values of what graphic designers use day to day. This would make the work of developers building their web design mockups much easier! 😄