add module field

Author: Feiyang1

packages/firestore/exp/package.json

@@ -2,6 +2,7 @@
   "name": "@firebase/firestore/exp",
   "description": "A tree-shakeable version of the Firestore SDK",
   "main": "../dist/exp/index.node.esm2017.js",
+  "module": "../dist/exp/index.browser.esm2017.js",
   "browser": "../dist/exp/index.browser.esm2017.js",
   "react-native": "../dist/exp/index.rn.esm2017.js",
   "typings": "../exp-types/index.d.ts",

packages/firestore/lite/package.json

@@ -2,6 +2,7 @@
   "name": "@firebase/firestore/lite",
   "description": "A lite version of the Firestore SDK",
   "main": "../dist/lite/index.node.esm2017.js",
+  "module": "../dist/lite/index.browser.esm2017.js",
   "browser": "../dist/lite/index.browser.esm2017.js",
   "react-native": "../dist/lite/index.rn.esm2017.js",
   "typings": "../lite-types/index.d.ts",