Add support for basic addVariant plugins with new @plugin directive

[adamwathan]

This PR adds support for a new @plugin directive as a first step towards reintroducing compatibility with v3 plugins:

@import "tailwindcss";
@plugin "my-plugin";

This PR only introduces support for the addVariant API for now, with support for the other plugin APIs planned for separate PRs.

Just like v3, plugins are JS files that look like this:

// my-plugin.js
module.exports = function ({ addVariant }) {
  addVariant('hocus', ['&:hover', '&:focus'])
}

This PR doesn't reintroduce the tailwindcss/plugin export yet that you can use to wrap plugins that support configuration options, but planning to reintroduce that as well to ensure old plugins continue to work.