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:

• Add tests
• Add docs
• 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 ✅

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Additional details and impacted files

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  Coverage          ?   88.18%           
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  Files             ?        5           
  Lines             ?      347           
  Branches          ?       91           
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  Hits              ?      306           
  Misses            ?       25           
  Partials          ?       16           

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