Bump rollup-plugin-postcss from 2.1.1 to 2.2.0

Bumps [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss) from 2.1.1 to 2.2.0.
- [Release notes](https://github.com/egoist/rollup-plugin-postcss/releases)
- [Changelog](https://github.com/egoist/rollup-plugin-postcss/blob/master/CHANGELOG-OLD.md)
- [Commits](egoist/rollup-plugin-postcss@v2.1.1...v2.2.0)

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