Bump webpack-dev-server from 3.7.2 to 3.9.0

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.7.2 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](webpack/webpack-dev-server@v3.7.2...v3.9.0)

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Author: dependabot-preview[bot]