[Improvement] Use Sass modules instead of global built-in functions

[TiLodoss]

First of all, thanks a lot for this lib, it's really nice and simple to use 🎉

While using it in a project that runs with Node 22 and sass 1.80, many deprecating warnings like (the one below) were fired.

Deprecation Warning: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use string.index instead.

More info and automated migrator: https://sass-lang.com/d/import

    ╷
132 │                 $index: str-index($string, $search);

I took the liberty to clone the project and I've fixed the warning ny using sass' modules. However, I am not able to create a PR since I'm not able to push a new branch. I'm kinda a contributor newbie 😅

Let me know if you would like to see the modifications I've made.

Cheers!