Replace node-sass with dart-sass

[schwemmer]

node-sass is giving a lot of problems when installing the IIIF manifest editor locally, especially when switching node environments (e.g. when testing different node versions with nvm) and when on Windows. node-sass then usually needs to be recompiled. The error messages are cryptic and not helpful. Also, the Sass project has switched to Dart, see here: https://sass-lang.com/blog/announcing-dart-sass