Update dependency file-loader to v4

Author: renovate-bot

playground/package-lock.json

@@ -26,7 +26,7 @@
     "css-loader": "2.1.0",
     "eslint": "5.14.0",
     "eslint-config-google": "0.12.0",
-    "file-loader": "3.0.1",
+    "file-loader": "4.1.0",
     "google-closure-compiler": "20190121.0.0",
     "handlebars": "4.0.12",
     "handlebars-loader": "1.7.0",