feat: upgrade mini-css-extract-plugin from 0.6.0 to 1.3.1

Snyk has created this PR to upgrade mini-css-extract-plugin from 0.6.0 to 1.3.1.

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

See this project in Snyk:
https://app.snyk.io/org/turkdevops/project/dbb9043e-c503-4a43-8370-905e7050ae27?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

packages/@vuepress/core/package.json

@@ -42,7 +42,7 @@
     "file-loader": "^3.0.1",
     "js-yaml": "^3.13.1",
     "lru-cache": "^5.1.1",
-    "mini-css-extract-plugin": "0.6.0",
+    "mini-css-extract-plugin": "1.3.1",
     "optimize-css-assets-webpack-plugin": "^5.0.1",
     "portfinder": "^1.0.13",
     "postcss-loader": "^3.0.0",