Replace node-sass with Dart Sass

[taneliang]

LibSass has been deprecated. Their maintainers recommend switching to Dart Sass instead.

Test plan: ran yarn start and yarn build; no apparent changes.

[vercel]

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nusmods-website – ./website

🔍 Inspect: https://vercel.com/nusmodifications/nusmods-website/fylg2kfrn
✅ Preview: https://nusmods-website-git-eliang-rip-node-sass.nusmodifications.vercel.app

nusmods-export – ./export

🔍 Inspect: https://vercel.com/nusmodifications/nusmods-export/quzpracd6
✅ Preview: https://nusmods-export-git-eliang-rip-node-sass.nusmodifications.vercel.app

[codecov]

Codecov Report

Merging #3197 (ad15c0b) into master (9dd0a85) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3197   +/-   ##
=======================================
  Coverage   55.44%   55.44%           
=======================================
  Files         254      254           
  Lines        5317     5317           
  Branches     1227     1227           
=======================================
  Hits         2948     2948           
  Misses       2369     2369           

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[nusmods-deploy-bot]

Deployment preview for ad15c0b2 ready at http://600fef71bc5445fc9832842b--nusmods-deploy-preview.netlify.app

[taneliang]

Merging without approval to unblock #3155

[ZhangYiJiang]

Is there a diff in the CSS output? I think that's the easiest way to check for regressions

[taneliang]

I did try to diff the generated CSS, but it was minified so it wasn't useful 😆 Anyway I figured it was unlikely that anything would break so I didn't pursue it further. This is what I did:

# Remove `contenthash` from `filename` and `chunkFilename` in `MiniCssExtractPlugin` config object in webpack.parts.js.

git checkout master
yarn && yarn build

mv dist dist-nodesass

git checkout eliang/rip-node-sass
yarn && yarn build

diff dist dist-nodesass