Package.json: Added "sideEffects": false

package.json

@@ -8,6 +8,7 @@
     "type": "git",
     "url": "https://github.com/mrdoob/three.js"
   },
+  "sideEffects": false,
   "files": [
     "build/three.js",
     "build/three.min.js",