chore(deps): bump unified from 8.4.1 to 8.4.2 (#371)

Bumps [unified](https://github.com/unifiedjs/unified) from 8.4.1 to 8.4.2.
- [Release notes](https://github.com/unifiedjs/unified/releases)
- [Changelog](https://github.com/unifiedjs/unified/blob/master/changelog.md)
- [Commits](unifiedjs/unified@8.4.1...8.4.2)

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