add postcss as a dev dependency

This allows us to run our tests!

Author: RobinMalfait

package.json

@@ -38,6 +38,7 @@
     "@swc/core": "^1.3.7",
     "@swc/jest": "^0.2.23",
     "jest": "^29.1.2",
+    "postcss": "^8.4.17",
     "prettier": "^2.7.1",
     "tailwindcss": "^0.0.0-insiders.4338849"
   }