add postcss-preset-env, remove autoprefixer

[heygrady]

https://github.com/csstools/postcss-preset-env

There is now a postcss plugin that attempts to do for CSS what babel-preset-env does for js. It reads the same browserlist as babel-preset-env. This makes it possible to write advanced CSS and have it compiled for your target browsers.

This PR removes autoprefixer because postcss-preset-env includes it. The end result is the exact same output as before but with more functionality. The big difference is that you can now do something crazy like use css vars with fallbacks for older browsers. (Using CSS vars instead of using SCSS has some real world advantages, like the ability to manipulate those vars with JS.)

:root {
  --background: #282c34;
}

.App-header {
  background-color: var(--background);
}

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need the corporate CLA signed.

If you have received this in error or have any questions, please contact us at cla@fb.com. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[Timer]

Can you update this please? :-)

[Timer]

Can we limit this to stage 4 only? We don't want to turn on anything stage 3 or less.

[Timer]

Awesome! Thanks for this. 😄