Bump webpack-dev-server from 3.4.1 to 3.9.0

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.4.1 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/v3.4.1...v3.9.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -19,7 +19,7 @@
     "webpack-merge": "^4.1.4"
   },
   "devDependencies": {
-    "webpack-dev-server": "3.4.1"
+    "webpack-dev-server": "3.9.0"
   },
   "scripts": {
     "webpack": "./bin/webpack",