[Feature]: Allow importing `unfonts.css` in the CSS

### Demo URL

_No response_

### Describe the feature

For custom fonts, we have to `import 'unfonts.css'` in JS, but that creates an extra CSS file / `<style>` tag; it would be nice to be able to import it directly in our existing CSS (with e.g. `@import 'unfonts.css'` – I tried that and it doesn't seem to work).