feat: adding default build scripts for react native example apps

Author: krispypen

bricks/react-native-example/__brick__/package.json

@@ -4,7 +4,9 @@
   "private": true,
   "scripts": {
     "android": "react-native run-android",
+    "build:android": "react-native build-android",
     "ios": "react-native run-ios",
+    "build:ios": "react-native build-ios",
     "lint": "eslint .",
     "start": "react-native start",
     "test": "jest"