Supports syntax highlighting features in *.md files

[87xie]

Description

In *.mdx files, syntax highlighting features work as expected, but they do not work in *.md files. I'm not sure if this is a bug or due to some limitations. The same situation occurs in both v2 and v3 alpha.

```js filename="test.js" {1}
console.log('hello world')

console.log('goodbye world')
```

*.mdx

*.md