sass: use dart-sass for building scss files

Michał Gołębiowskirequested to merge
mgolebiowski/rebuilderd-website:scss/move-to-dart-sass into master

This PR refactors the style.scss to use @use clause instead of @import, fixes #7

The @use clause is only implemented in dart sass implementation which will be now used for CI and should be used for local development.

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