* add prettier-plugin-tailwindcss
This will use the prettier plugin in our tests as well, yay consistency!
* ensure that both `group` and `peer` can't be used in `@apply`
This was only configured for `group`
* expose `sortClassList` on the context
This function will be used by the `prettier-plugin-tailwindcss` plugin,
this way the sorting happens within Tailwind CSS itself adn the
`prettier-plugin-tailwindcss` plugin doesn't have to use internal /
private APIs.
The signature looks like this:
```ts
function sortClassList(classes: string[]): string[]
```
E.g.:
```js
let sortedClasses = context.sortClassList(['p-1', 'm-1', 'container'])
```
* update changelog
* add sort test for utilities with the important modifier e.g.: `!p-4`
