Broken yaml code block syntax highlighting

[s-weigand]

First of all thanks for the awesome extension 😄

When trying it out I found that the syntax highlighting for yaml is broken.
For example this code snipped

```yaml
foo:
    bar: [1,2]
```

Without the extension, it renders like this

And after installing the extension

Html markup without extension

Html markup with extension

Maybe there is a way to tell markdown-it to ignore yaml and yml?

Other code blocks like python and js work normally.

[agoose77]

I can reproduce this, but I'm not initially sure what the cause is. jupyterlab-markup is receiving the highlight request, and passing it off to CM5, but I think something is going wrong thereafter.

[bollwyvl]

This is actually the case for any mode which hasn't been loaded yet (a few are blessed, e.g. JSON, markdown, python). If it's loaded before the render occurs, it should work: for example, creating a .yml file and opening it with the editor, then re-opening a markdown file which has a yaml code block looks correct.

A quick look upstream suggests, after determining which mode is needed, we need to call await Mode.ensure(spec)... somewhere around here... which of course we can't have, directly, because asynchrony.