Bump rollup-plugin-postcss from 2.9.0 to 3.1.0

Bumps [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss) from 2.9.0 to 3.1.0.
- [Release notes](https://github.com/egoist/rollup-plugin-postcss/releases)
- [Changelog](https://github.com/egoist/rollup-plugin-postcss/blob/master/CHANGELOG-OLD.md)
- [Commits](https://github.com/egoist/rollup-plugin-postcss/commits/v3.1.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

package.json

@@ -71,7 +71,7 @@
     "rollup": "^2.0.2",
     "rollup-plugin-babel": "^4.3.3",
     "rollup-plugin-local-resolve": "^1.0.7",
-    "rollup-plugin-postcss": "^2.1.1",
+    "rollup-plugin-postcss": "^3.1.0",
     "rollup-plugin-string": "^3.0.0",
     "stylelint": "^13.0.0",
     "stylelint-config-standard": "^20.0.0",