Bump postcss-loader from 2.1.6 to 3.0.0

Bumps [postcss-loader](https://github.com/postcss/postcss-loader) from 2.1.6 to 3.0.0.
- [Release notes](https://github.com/postcss/postcss-loader/releases)
- [Changelog](https://github.com/postcss/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss-loader/compare/v2.1.6...v3.0.0)

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

package.json

@@ -35,7 +35,7 @@
     "imagemin-webpack-plugin": "^1.6.1",
     "node-sass": "^4.13.0",
     "postcss-cssnext": "^3.0.2",
-    "postcss-loader": "^2.0.8",
+    "postcss-loader": "^3.0.0",
     "sass-loader": "^6.0.6",
     "shx": "^0.2.2",
     "style-loader": "^0.19.0",