build: use watch option to auto rebuild dependency

Just want to make development convenient

Author: swsoyee

inst/templates/package.json.txt

@@ -12,5 +12,9 @@
     "@babel/preset-react": "^7.0.0",
     "css-loader": "^5.0.1",
     "style-loader": "^2.0.0"
+  },
+  "scripts": {
+    "watch": "webpack --watch",
+    "build": "webpack"
   }
 }