chore: add prepublishOnly

Make sure we always build before publishing.

Author: tkurki

package.json

@@ -22,6 +22,7 @@
   "repository": "github:SignalK/instrumentpanel.git",
   "scripts": {
     "start": "node server.js",
+    "prepublishOnly": "npm run build",
     "build": "webpack -p --progress",
     "build-debug": "webpack -d --devtool cheap-module-source-map --progress",
     "watch": "webpack -d --watch",