Use sass (Dart SASS) instead of node-sass

As it becomes more and more troublesome to install node-sass on modern platforms and node-sass is deprecated anyway, we now switch to the sass package which is a pure JS Sass compiler.

There are some possible issues with existing projects:

• The performance is a little worse for larger Sass files
• There might will be incompatibilities, one that we found while testing was:
  • Using Sass Syntax in .css files was apparently ok with node-sass but sass throws an error in these cases