fix mdx-code-block special code block for docusaurus

[cderv]

It is not handled by docusaurus if there is whitespace after backtick like pandoc is creating from CodeBlock

This fixes #8333

Though from discussion there, it could also be an issue in docusaurus itself that they should fix as it is breaking from V2 to V3
and it does not seem they mention this change as known (cc @eitsupi)

Their checker (https://github.com/slorber/docusaurus-mdx-checker) does not warn on the produced file with a space also

So opening as draft to keep this if we decide to merge it

[cscheid]

I think we need to merge this because we're providing the translation of blocks like this into mdx-code-block blocks as a feature:

```{=mdx}
...
```

We don't want Quarto to regress regress here, independently of how brittle Docusaurus is on this respect.

[cderv]

I agree, that is why I open the PR. I'll undraft and let you merge if we go for it for 1.4

I did not add a new tst by the way, but just adapted the existing one.