chore(package): update core-js to version 3.1.2

Closes #22

Author: greenkeeper[bot]

package.json

@@ -65,7 +65,7 @@
     "commitplease": "3.2.0",
     "conventional-github-releaser": "2.0.2",
     "copy-webpack-plugin": "^5.0.2",
-    "core-js": "^2.6.0",
+    "core-js": "^3.1.2",
     "css-loader": "^2.1.1",
     "cssnano": "^4.1.7",
     "del": "3.0.0",