message: [Improvement] packages.json add module entry
diff: "name": "vant",
  "version": "1.0.8",
  "description": "A Vue.js 2.0 Mobile UI at YouZan",
  - "main": "lib/vant.js",
  + "main": "lib/index.js",
  + "module": "es/index.js",
  "style": "lib/vant-css/index.css",
  - "unpkg": "lib/index.js",
  "typings": "types/index.d.ts",
  "files": [
  - "lib",
  "es",
  + "lib",
  "packages",
  "types"
  ],
  