perf: Purge Tailwind CSS for Storybook

Current code base doesn't purge CSS for storybook:
https://github.com/downwindcss/easings/blob/main/tailwind.config.js#L2

```js
module.exports = {
  purge: [], // <--- Add purge regex here
  darkMode: false, // or 'media' or 'class'
  // rest of configuration...
};
```