Add support for postcss using postcss-preset-env

[szimek]

Is this a bug report?

No.

It would be really great if create-react-app had support for postcss plugins. Recently, it became much easier to do with postcss-preset-env, which is very similar in configuration to @babel/preset-env, i.e. one can specify which browsers should be supported (the package also supports browserslist) and which features (either whole stages or individual features or mix of those) - https://preset-env.cssdb.org/. It also already includes autoprefixer.

The popular postcss-cssnext package was deprecated in favor of postcss-preset-env.

[ianschmitz]

There's a PR over at #4716

[szimek]

Awesome! I'll close this one then. I really hope this PR gets merged.