refactor!: update to PostCSS 8 (#151)

Fixes: #150

* chore(deps): bump `postcss` to latest version.
* chore(deps): bump `postcss-parser-tests` to latest version.
* refactor(tests): use new `postcss-parser-tests` exports.

BREAKING CHANGES: drop Node 8 support, as PostCSS 8 requires Node >= 10.

Author: ludofischer