Update dependency mini-css-extract-plugin to v0.9.0 (#4000)

Author: renovate-bot

package-lock.json

@@ -92,7 +92,7 @@
     "grunt-wp-deploy": "2.0.0",
     "jest-silent-reporter": "0.1.2",
     "lodash": "4.17.15",
-    "mini-css-extract-plugin": "0.8.2",
+    "mini-css-extract-plugin": "0.9.0",
     "moment": "2.24.0",
     "npm-run-all": "4.1.5",
     "null-loader": "3.0.0",