Use PostCSS through Node for Sass compilation

[aduth]

Why:

• To consolidate fragmented build apparatus
• To improve build performance†, where Ruby-based autoprefixer-rails is doing the same work, but by a process spawned through Ruby via ExecJS
  • † This benefit is likely limited to development. Since this does add a new PostCSS plugin where one did not exist previously (CSSNano), there could be a net-increase in build time for production-optimized builds.
• To have more control over dependency tree to use the latest version
  • Current latest autoprefixer-rails uses outdated autoprefixer@10.4.2, where current latest Autoprefixer is autoprefixer@10.4.4
• Allows us to (re-?)integrate CSSNano plugin for optimized output

While we already have basic CSS minification through Sass's "compressed" style output, CSSNano includes quite a few more optimizations that transform the output, while Sass's compressed output is largely concerned with omitting additional whitespace.

Performance:

RAILS_ENV=production rake assets:precompile && \
  du -sh public/assets/application-[hash].css && \
  gzip-size public/assets/application-[hash].css && \
  brotli-size public/assets/application-[hash].css

Compression	Before	After	Diff	Diff%
Raw	392kb	348kb	-44kb	-11.2%
Gzip	53.9kb	48kb	-5.9kb	-10.9%
Brotli	40.3kb	36.7kb	-3.6kb	-8.9%

[mitchellhenke]

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