Bump webpack-dev-server from 3.11.3 to 4.15.1

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.11.3 to 4.15.1.
- [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@v3.11.3...v4.15.1)

---
updated-dependencies:
- dependency-name: webpack-dev-server
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

package.json

@@ -70,7 +70,7 @@
     "webpack-cli": "^3.3.12"
   },
   "devDependencies": {
-    "webpack-dev-server": "^3"
+    "webpack-dev-server": "^4"
   },
   "homepage": "https://github.com/publiclab/plots2",
   "engines": {