replace node-sass with dart-sass

node-sass was charming, since it was using a faster c based compiler (libsass).

however, the project got deprecated: https://www.npmjs.com/package/node-sass/v/5.0.0

"Warning: LibSass and Node Sass are deprecated. While they will continue to receive maintenance releases indefinitely, there are no plans to add additional features or compatibility with any new CSS or Sass features. Projects that still use it should move onto Dart Sass."

i tested the now called sass package (dart-sass) and the theme builds work fine.

the package is also now included officially: https://sass-lang.com/install