Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.8.1 to 3.9.0.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack/webpack-dev-server/compare/v3.8.1...v3.9.0

)

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