Update dependency css-loader to v6

Author: renovate-bot

package.json

@@ -21,7 +21,7 @@
     "babel-plugin-import": "1.13.0",
     "clean-webpack-plugin": "3.0.0",
     "copy-webpack-plugin": "5.1.1",
-    "css-loader": "3.5.3",
+    "css-loader": "6.4.0",
     "extract-text-webpack-plugin": "4.0.0-beta.0",
     "file-loader": "6.0.0",
     "html-webpack-plugin": "4.3.0",