refactor: postcss is a peer dependency

Author: mfranzke

package-lock.json

@@ -52,6 +52,7 @@
     "nodemon": "3.1.10",
     "npm-run-all2": "8.0.2",
     "plugin-node-patternlab-inline-remote-assets": "^0.2.1",
+    "postcss": "^8.5.3",
     "prettier": "3.5.3",
     "sass": "^1.89.0",
     "style-dictionary": "3.9.2",