Add bag of variants

[ben-rogerson]

This pr adds the following variants featured in tailwindcss@2.2.2:

• autofill:
• in-range:
• out-of-range:
• first-letter:
• first-line:
• only:
• empty:
• selection:
• marker:

[lightyen]

• These group-* are not supported. https://github.com/tailwindlabs/tailwindcss/blob/b0128bfe0a3798365278da4000795dc229bbd8bf/tests/jit/variants.test.html

• The only-child should be deprecated.

- only-child
+ only

- not-only-child
+ not-only

[ben-rogerson]

Good pickups, I'll add those group classes and move to only instead of only-child 👍