Support variant as functions

[AdrienLemaire]

The issue https://github.com/tailwindlabs/tailwindcss/pull/2309 has been released in v1.8, and I'm getting this error:

> tailwindcss-classnames --config src/utilities/tailwind.js -o src/utilities/tailwind.ts

(node:755354) UnhandledPromiseRejectionWarning: TypeError: variantsForKey.map is not a function

Code changed:

  variants: {
-    fontSize: ["responsive", "hover", "focus"],
+    fontSize: ({ after }) => after(["hover", "focus"]),
  }