[Bug] Mermaid diagrams fails to show on Playground on milkdown.dev

[michaelcychan]

Initial checklist

• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Affected packages and versions

v7.5.9

Link to runnable example

No response

Steps to reproduce

1. visit https://milkdown.dev/playground
3. Add the following mermaid snippet to the raw markdown text area:

```mermaid
flowchart TD
A[Christmas] -->|Get money| B(Go shopping)
B --> C{Let me think}
C -->|One| D[Laptop]
C -->|Two| E[iPhone]
C -->|Three| F[fa:fa-car Car]
```

Expected behavior

a mermaid diagram should be rendered on the preview side:

Actual behavior

The mermaid snippet appears as a normal code snippet

Runtime

Chrome

OS

macOS

Build and bundle tools

Other (please specify in steps to reproduce)