add addVariant shorthand signature

The current API looks like this:

```js
addVariant('name', ({ format, wrap }) => {
  // Wrap in an atRule
  wrap(postcss.atRule({ name: 'media', params: '(prefers-reduced-motion: reduce)' }))

  // "Mutate" the selector, for example prepend `.dark`
  format('.dark &')
})
```

It is also pretty common to have this:
```js
addVariant('name', ({ format }) => format('.dark &'))
```
So we simplified this to:
```js
addVariant('name', '.dark &')
```

It is also pretty common to have this:
```js
addVariant('name', ({ wrap }) => wrap(postcss.atRule({ name: 'media', params: '(prefers-reduced-motion: reduce)' })))
```
So we simplified this to:
```js
addVariant('name', '@media (prefers-reduced-motion: reduce)')
```

Author: RobinMalfait

src/lib/setupContextUtils.js

@@ -186,7 +186,18 @@ function buildPluginApi(tailwindConfig, context, { variantList, variantMap, offs
 
   return {
     addVariant(variantName, variantFunctions, options = {}) {
-      variantFunctions = [].concat(variantFunctions)
+      variantFunctions = [].concat(variantFunctions).map((variantFunction) => {
+        if (typeof variantFunction !== 'string') {
+          return variantFunction
+        }
+
+        if (!variantFunction.startsWith('@')) {
+          return ({ format }) => format(variantFunction)
+        }
+
+        let [, name, params] = /@(.*?) (\(.*\))/g.exec(variantFunction)
+        return ({ wrap }) => wrap(postcss.atRule({ name, params }))
+      })
 
       insertInto(variantList, variantName, options)
       variantMap.set(variantName, variantFunctions)