code block rendering bug for square bracket

Code block with triple backticks is rendering bad:

```ts
interface MyObject {
[key: string]: string;
}
```

No source file or test is named. Reproduce the issue in MacDown using the TypeScript interface inside a triple-backtick code block, then trace the Markdown rendering entry point. Done means square-bracket index signatures render as code without malformed output.

Written by the indexing model from the issue text.