Migrate to Dart Sass from LibSass

[jon-kirwan]

What

Migrate to Dart Sass from LibSass.

Why

• LibSass is now deprecated
• Making changes ahead of Design System team dropping support for LibSass Dropping support for Ruby Sass and LibSass govuk-frontend#2632.

Visual Changes

None.

How can I test these changes?

• run yarn jasmine:prepare (or run bundle exec rake app:assets:clobber app:assets:precompile)
• what should happen? The Sass files should be compiled. Keep an eye on the command line for warnings indicating the compilation process using Dart Sass

Deprecation Warning: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.
Recommendation: math.div(300%, 4) or calc(300% / 4)
More info and automated migrator: https://sass-lang.com/d/slash-div
   │
26 │   three-quarters: (300% / 4),
   │                    ^^^^^^^^
   ...

• the /builds folder: app/assets/builds should now be populated with CSS files. Note, the
• subsequently, all assets ought to be generated successfully within the /public directory
• no test failures to indicate the correct number of stylesheets are generated
• deploy to Integration. You'll notice that in the 'build and publish image' task, the steps taken resemble those mentioned earlier, dartsass-rails is bundled and any Dart Sass warnings encountered during the compilation process are reported)
• inspect several pages, e.g.
  guide, detailed guide, HTML publication, news article, specialist document, statistical dataset, foreign travel advice, worldwide organisation to ensure the absence of errors.

Anything else

Once the PR gets the green light, any temporary commits and references to local gems e.g. gem "govuk_publishing_components", path: "../govuk_publishing_components" will be removed.

See:

• Migrate to Dart Sass from LibSass collections#3447
• Migrate to Dart Sass from LibSass govuk_publishing_components#3726
• Migrate to Dart Sass from LibSass static#3190
• Migrate to Dart Sass from LibSass govuk-developer-docs#4216.

[matthillco]

Tested locally and on integration. Looks good, no errors or warnings. Nice job.