build(deps-dev): bump webpack-dev-server from 3.11.2 to 4.0.0 (#342)

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.11.2 to 4.0.0.
- [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@v3.11.2...v4.0.0)

---
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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Osamu Takiya <takiya@toran.sakura.ne.jp>

package.json

@@ -16,6 +16,6 @@
     "postcss-import": ">=14.0.2",
     "postcss-loader": ">=5.3.0",
     "sugarss": ">=3.0.3",
-    "webpack-dev-server": ">=3.11.2"
+    "webpack-dev-server": ">=4.0.0"
   }
 }