Add a root "dev" script

Author: kraenhansen

package.json

@@ -9,7 +9,8 @@
   ],
   "homepage": "https://github.com/callstackincubator/react-native-node-api-modules#readme",
   "scripts": {
-    "build": "tsc --build"
+    "build": "tsc --build",
+    "dev": "tsc --build --watch"
   },
   "author": {
     "name": "Callstack",