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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Please upload report for BASE (master@4a5e195). Learn more about missing BASE report. Report is 15 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #80 +/- ## ========================================= Coverage ? 88.18% ========================================= Files ? 5 Lines ? 347 Branches ? 91 ========================================= Hits ? 306 Misses ? 25 Partials ? 16 ```

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