Underscores should not italicize inside of a code block in Markdown

[ethanblake4]

When displaying a code block in Markdown, the pub.dev site italicizes text following an underscore character, which should only happen outside of a code block. However, it does (correctly) display the underscore character as well.

To reproduce: Publish a package with a readme containing:

```
├── test_one
│   └── test2
│       └── test_three
```

[isoos]

@ethanblake4: do you have a package that on pub.dev renders incorrectly? I've run the example through the backend's markdown rendering and it seems to be correct.

[ethanblake4]

https://pub.dev/packages/flutter_eval
search page for "bindings"

[isoos]

Thanks for the clarification! This seems to be an issue in highlightjs, I will check if it has been already reported and/or fixed in it.

[isoos]

Reported in highlightjs/highlight.js#4279 (comment)