[Feature] Add mermaid preview (#207)

Author: lzm0x219

package.json

@@ -149,7 +149,8 @@
     "markdown-it-footnote": "3.0.3",
     "markdown-it-github-headings": "2.0.1",
     "markdown-it-highlightjs": "4.0.1",
-    "markdown-it-task-lists": "2.1.1"
+    "markdown-it-task-lists": "2.1.1",
+    "mermaid": "10.2.2"
   },
   "devDependencies": {
     "@types/markdown-it": "12.2.3",