Add "sideEffects": false to package.json (#941)

Add `"sideEffects": false` to package.json to allow frameworks that mix client and server code in the same files to tree-shake out fs-extra from client bundles.

package.json

@@ -62,5 +62,6 @@
     "test-find": "find ./lib/**/__tests__ -name *.test.js | xargs mocha",
     "test": "npm run lint && npm run unit",
     "unit": "nyc node test.js"
-  }
+  },
+  "sideEffects": false
 }