Switch to dart-sass

[boncey]

I had some problems getting dashbling to work on an Apple Silicon Mac.
It turns out that node-sass doesn't support the ARM64 architecture. Digging deeper I found that LibSass is deprecated and they recommend switching to dart-sass.

Hopefully it's simple enough.

If you’re a user of Node Sass, migrating to Dart Sass is straightforward: just replace node-sass in your package.json file with sass. Both packages expose the same JavaScript API.