Bump webpack-merge from 4.1.5 to 4.2.1

Bumps [webpack-merge](https://github.com/survivejs/webpack-merge) from 4.1.5 to 4.2.1.
- [Release notes](https://github.com/survivejs/webpack-merge/releases)
- [Changelog](https://github.com/survivejs/webpack-merge/blob/master/CHANGELOG.md)
- [Commits](https://github.com/survivejs/webpack-merge/compare/v4.1.5...v4.2.1

)

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