fix: use named MarkdownIt type import for markdown-it v15 types

Update markdown-it to v15.0.3 and mime to v4.1.1.

Fixes #21

tsconfig.json

@@ -9,7 +9,7 @@
     "allowJs": true,
     "noEmit": true,
     "rootDir": ".",
-    "skipLibCheck": true,
+    "skipLibCheck": false,
     "strict": true,
     "allowUnusedLabels": false,
     "allowUnreachableCode": false,