Detect language within fenced code block when echo: fenced

[cderv]

Detect language within fenced code block when echo: fenced is used for code annotationWelcome to the quarto GitHub repo!

closes #6244

This suppose that when echo: fenced there won't be any language on cell and so only class found will be cell-code. In that case we extract from the text content of CodeBlock the language between ```{{lang}}

We could also try to apply the regex no matter the language on the CodeBlock but it felt too wide...

This PR also adds support for HTML and Markdown code cells using HTML comments as comment chars.

[dragonstyle]

🔥

[dyavorsky]

@cderv As of 2024.04.02 on Quarto version 1.4.552, I don't believe the support for markdown cells in Code Annotation has been fixed.

In the following example, code annotation does not work when the language is "markdown" (or "html" or "latex") but it does work when the language is most other things, eg "r"

```markdown
# Header        # <1>
This is a test  # <2>
```
1. a header
2. some content

[cderv]

The right comment chars needs to be used. This works

```markdown
# Header       <!-- <1> -->
This is a test  <!-- <2> -->
```
1. a header
2. some content

as in the tests shown in this PR.

HTML use also HTML comment.
LaTeX uses % so this will work

```latex
\usepackage{graphics}  % <1>
```
1. a header

[dyavorsky]

This make sense. Very helpful -- I appreciate the fast reply! Can I buy you a coffee as a apology for calling you out? :)