Bump postcss-nested from 4.1.2 to 4.2.1

Bumps [postcss-nested](https://github.com/postcss/postcss-nested) from 4.1.2 to 4.2.1.
- [Release notes](https://github.com/postcss/postcss-nested/releases)
- [Changelog](https://github.com/postcss/postcss-nested/blob/master/CHANGELOG.md)
- [Commits](postcss/postcss-nested@4.1.2...4.2.1)

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