PostCSS JS config breaks Vitest

[goosewobbler]

See vitest-dev/vitest#4201.

The workaround was to convert the PostCSS config to JSON but this changes the use of cssnano as it was behind a check for the production build:

if (process.env.NODE_ENV === 'production') {
  plugins.push(
    cssnano({
      preset: 'default',
    }),
  );
}