fix: upgrade mini-css-extract-plugin from 0.9.0 to 0.12.0 (#8)

Snyk has created this PR to upgrade mini-css-extract-plugin from 0.9.0 to 0.12.0.

See this package in npm:
https://www.npmjs.com/package/mini-css-extract-plugin

See this project in Snyk:
https://app.snyk.io/org/j-vin-b8w/project/6912bb9d-a3bd-4a54-bbc5-d47aece5b486?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package.json

@@ -67,7 +67,7 @@
     "lunr": "^2.3.9",
     "lunr-languages": "^1.4.0",
     "mdast-util-from-markdown": "^0.8.4",
-    "mini-css-extract-plugin": "^0.9.0",
+    "mini-css-extract-plugin": "^0.12.0",
     "mkdirp": "^1.0.4",
     "morgan": "^1.10.0",
     "node-fetch": "^2.6.1",