🤺 Amenable Fences

[zampino]

The purpose of these changes is two-fold:

• Help solving annoying issues with the undesired evaluation of code in markdown fenced code blocks (currently always treated as clojure code meant for evaluation) like this repo's README.md. Also introduces a skip or eval=false (inspired by RMarkdown/KnitR) annotaions to be used to signal a clojure fenced block should not be evaluated:

```clojure {:nextjournal.clerk/code-listing true}
'(1 2 "not evaluated" :but-still-highlighted)
```

• Prepare for highlighting inert markdown fenced code with suitable prosemirror styles. Don't highlight languages other than clojure for the moment.

[mk]

I don't really like skip as I think it's ambiguous. eval=false is much clearer but then I also don't love that the default is true.

Looking at prior art, quarto seems to use eval=false under the execute key in all its markdown flavors:

• https://quarto.org/docs/reference/formats/markdown/gfm.html#execution
• https://quarto.org/docs/reference/formats/markdown/commonmark.html#execution
• https://quarto.org/docs/reference/formats/markdown/markua.html#execution

Ignoring this prior art (which I'm not sure we should do) I think I'd prefer something like no-eval=true or code-listing=true. In general I think it's good that we give some thought to how we want to support other clerk attributes to enable foldable code-listings or controlling width for example in languages that aren't clojure.

[mk]

Leaning towards edn in the annotations so:

```javascript {:nextjournal.clerk/visibility {:code :fold} :nextjournal.clerk/code-listing true}
41 + 1
```

Potentially dropping the namespace step and namespacing programmatically:

```javascript {:visibility {:code :fold} :code-listing true}
41 + 1
```

Which we then namespace so {:nextjournal.clerk/visibility {:code :fold} :nextjournal.clerk/code-listing true}.

If we want to support a short-hand like for Clojure metadata we could read multiple times and merge, i.e.

```javascript :code-listing {:visibility {:code :fold}}
41 + 1
```

For now I think I'm leaning towards the first option. A bit longer, but nice, simple & explicit.