[Remove Node.js] Use dart-sass for SCSS

[malparty]

inherit #431

[!NOTE] This issue does NOT include any code change in the Rails template (yet), as these will be based on the RFC results.

Why

SCSS build currently relies on Node.js.

Checklist

• [x] POC in CX (side branches)
• [ ] Presentation of learnings (WIP in https://docs.google.com/presentation/d/1BOXkg7wGwSkzdqILP3VH7XDiXNDsXiR719jumeICktQ/edit#slide=id.gf7533c37e2_3_0 )
• [ ] RFC in GitHub discussion

Insight 📝

The cssbundling-rails gem relies on Node.js. [Rails Guides](https://guides.rubyonrails.org/asset_pipeline.html#dartsass-rails) recommend using the dartsass-rails to remove Node.js dependency.

./bin/bundle add dartsass-rails
./bin/rails dartsass:install

[!NOTE] It turns out import-maps is limited to JS and won’t let you pin other assets like SCSS or images. https://github.com/rails/importmap-rails/issues/80

“If you need Node for anything, I'd use it for everything. The benefit of import maps is that it provides a path with no Node at all. cssbundling-rails and jsbundling-rails is a great combo to use with Bootstrap 👍” DHH – [source](https://github.com/rails/importmap-rails/issues/107#issuecomment-1049793920)

Who Benefits?

• Developers for the knowledge
• Future clients' projects for improved tooling decisions.

[malparty]

Issue started in https://github.com/nimblehq/cx-web/pull/757