[IMP] package: only watch static file

Currently, every file is watched by the static server.
including typescript files and intermediary js files.
But the static server only cares about the bundle file
and demo files.

Part-of: #2200

package.json

@@ -11,7 +11,7 @@
     "dist/*.d.ts"
   ],
   "scripts": {
-    "serve-static": "live-server --open=demo --watch=build,demo",
+    "serve-static": "live-server --open=demo --watch=build/o_spreadsheet.js,demo",
     "dev": "npm-run-all --print-label build --parallel server serve-static watch:*",
     "server": "node tools/server/main.js",
     "build:js": "tsc --module es6 --incremental",