Manually building `public/css/style.css` using latest `sass` release fails because of `_mixins.css` not using `.scss` extension
I am currently setting up nitter on a system using Oracle Linux 9.
It lacks the `libsass` package so `nimble scss` fails because it doesnt find the library (obviously).
Grabbing the latest standalone release of `sass` from https://github.com/sass/dart-sass as recommended by https://sass-lang.com/install/ and replacing `nimble.scss` with `sass src/sass/index.scss public/css/style.css --load-path=src/sass/include` fails as follows:
```
Error: This at-rule isn't allowed in plain CSS.
  ╷
3 │ @mixin panel($width, $max-width) {
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ╵
  _mixins.css 3:1                @import
  src/sass/tweet/_base.scss 2:9  @import
  src/sass/index.scss 3:9        root stylesheet
```

I noticed that `_mixins.css` was not using the `.scss` extension.
After a `mv src/sass/include/_mixins.css src/sass/include/_mixins.scss` the build works just fine.

The sass webpage suggests that libsass is deprecated and was lagging behind the other implementations as well. I assume that libsass working just fine with the "wrong" extension is one example of this.