Highlight specific lines does not work anymore / 'highlight' class not added

[PatrikBird]

Hello and thanks for your incredible work on nuxt/content ❤️

I think I came across a bug in this module today.
Highlighting specific lines of code does not work anymore (since nuxt/content 2.8.0)

```vue {2,6-10}
using this syntax in .md files should add a class to the lines 2 and 6-10 that can be used to style those lines
```
I attached two minimal reproductions to explain it further.

Expected behaviour: Using nuxt 3.6 and nuxt/content 2.7.2

You can see that the code highlighting for the specific lines works as expected.

html output with nuxt/content 2.7.2

Reproduction: https://stackblitz.com/edit/nuxt-content-v2-highlight-class?file=package.json

---

Unexpected behaviour: Using nuxt 3.6 and nuxt/content 2.8.0

You can see that the highlights disappeared using nuxt/content 2.8.0.

html output with nuxt/content 2.8.0

Reproduction: https://stackblitz.com/edit/nuxt-content-v2-highlight-class-qb55kd?file=package.json

Further information

• nuxt/content 2.8.0 was the first version that uses this module, therefore I created this bug here.
• I tried the newest version (2.8.2) as well, same issue.