bump(postcss): Support postcss v8

[kharrop]

What Changed

Following the migration guide for https://evilmartians.com/chronicles/postcss-8-plugin-migration, this change requires libraries to use postcss v8.

Why

Certain plugins rely on postcss v8; bumping this in postcss-themed allows use of this plugin in those environments.

Todo:

• [x] Add tests
• [x] Add docs
• [x] Add yourself to contributors (run yarn all-contributors add [name] [type]) see https://github.com/all-contributors/all-contributors/blob/master/docs/cli/usage.md

Published PR with canary version: 3.0.0-canary.80.879