Switch from PostCSS/Autoprefixer/CSSNano to Parcel CSS

[aduth]

Why:

• Improved performance, both at build time and runtime.
• Fewer dependencies to manage and maintain

I had tried this once before and the feature support with autoprefixing didn't have what we needed at the time (especially CSS mask used by a lot of USWDS icons), but it looks like this has since improved. Will be worth testing a bit more exhaustively to check overall support.

I also want to confirm more closely that the browserlist configuration is being used as expected.

Resources:

• https://github.com/parcel-bundler/parcel-css

Impact:

yarn build:css (development bundling)

Before: ~3.5s
After: ~2.2s

NODE_ENV yarn build:css (production bundling with minification)

Before: ~5.3s
After: ~2.5s

NODE_ENV yarn build:css && brotli-size app/assets/builds/application.css (production stylesheet size)

Before: 36.6kb
After: 36.0kb

[aduth]

I gave this a thorough shake locally, spot-checking individual code differences (particularly on vendor prefixes like -webkit-), and went through critical flows across primary supported browsers.

Next I'm going to test this in my personal sandbox to make sure the binaries don't give us any trouble in deployment, but otherwise this is shaping up nicely to merge.

Edit: Looks good in my personal sandbox!