Update sideEffects property for webpack 4

Author: wojtekmaj

package.json

@@ -5,7 +5,9 @@
   "main": "dist/umd/index.js",
   "module": "dist/esm/index.js",
   "source": "src/index.js",
-  "sideEffects": false,
+  "sideEffects": [
+    "*.css"
+  ],
   "scripts": {
     "build": "yarn build-js-all && yarn copy-styles && yarn build-styles",
     "build-js-all": "yarn build-js-esm && yarn build-js-umd",