examples: add preinstall script

examples/nodejs/package.json

@@ -1,6 +1,9 @@
 {
   "type": "module",
   "main": "index.js",
+  "scripts": {
+    "preinstall": "cargo run --bin bundle_pubky_npm"
+  },
   "dependencies": {
     "@synonymdev/pubky":"file:../../pubky/pkg" 
   }