Bump css-vars-ponyfill from 2.1.2 to 2.2.1

Bumps [css-vars-ponyfill](https://github.com/jhildenbiddle/css-vars-ponyfill) from 2.1.2 to 2.2.1.
- [Release notes](https://github.com/jhildenbiddle/css-vars-ponyfill/releases)
- [Changelog](https://github.com/jhildenbiddle/css-vars-ponyfill/blob/master/CHANGELOG.md)
- [Commits](https://github.com/jhildenbiddle/css-vars-ponyfill/commits

)

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