Bump @fullhuman/postcss-purgecss from 2.1.0 to 3.0.0

Bumps [@fullhuman/postcss-purgecss](https://github.com/FullHuman/purgecss) from 2.1.0 to 3.0.0.
- [Release notes](https://github.com/FullHuman/purgecss/releases)
- [Changelog](https://github.com/FullHuman/purgecss/blob/master/CHANGELOG.md)
- [Commits](FullHuman/purgecss@v2.1.0...v3.0.0)

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Author: dependabot-preview[bot]