feat(mdx)!: support File Code Block of ```tsx file="./filename" to write demo code in another file

[SoonIter]

Summary

feat: File Code Block of ```tsx file="./filename"

Users can import an external file as a code block.

As we support this feature in the core. This also means that the syntax external code blocks are used in plugin-preview and plugin-playground will change.

before

<code src="./example.tsx" />

after

```tsx file="./example.tsx"
```

Related Issue

close #2182

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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