chore(renovate): group pinia packages (#3032)

Group `pinia`, `@pinia/nuxt`, and `pinia-plugin-persistedstate` into a single Renovate group so they update together.

Reviewed-on: https://gitea.unbound.se/dancefinder/dancefinder-app/pulls/3032

renovate.json

@@ -10,6 +10,14 @@
         "@grafana/faro-**"
       ]
     },
+    {
+      "groupName": "pinia",
+      "matchPackageNames": [
+        "pinia",
+        "@pinia/nuxt",
+        "pinia-plugin-persistedstate"
+      ]
+    },
     {
       "groupName": "stylelint",
       "matchPackageNames": [