chore(deps-dev): bump rollup-plugin-postcss from 3.1.5 to 3.1.8

Bumps [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss) from 3.1.5 to 3.1.8.
- [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/compare/v3.1.5...v3.1.8)

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

package.json

@@ -59,7 +59,7 @@
     "rollup-plugin-livereload": "^1.3.0",
     "rollup-plugin-node-resolve": "^5.2.0",
     "rollup-plugin-peer-deps-external": "^2.2.3",
-    "rollup-plugin-postcss": "^3.1.5",
+    "rollup-plugin-postcss": "^3.1.8",
     "rollup-plugin-replace": "^2.2.0",
     "rollup-plugin-serve": "^1.0.4",
     "rollup-plugin-terser": "^7.0.1",