update terser-webpack-plugin to v 2.2.1 (#440)

see: https://support.squidex.io/t/can-not-build-with-terser-webpack-plugin-v2-1-2/1307

frontend/package.json

@@ -89,7 +89,7 @@
     "sass-lint": "1.13.1",
     "sass-loader": "8.0.0",
     "style-loader": "1.0.0",
-    "terser-webpack-plugin": "2.1.2",
+    "terser-webpack-plugin": "2.2.1",
     "ts-loader": "6.2.0",
     "tsconfig-paths-webpack-plugin": "3.2.0",
     "tslint": "5.20.0",