Add support for external renders for code blocks

[davidsvantesson]

Just as you can specify an external render for a specific file, it would be cool to have an external render for a specific code block. This should be handled backend and the data feed to the external script would be the content of the code block instead of a complete file.

Code highligtning is currently handled frontend by javascript. Placing the external tool support in the backend would allow more possibilities on what is possible to do as it is not requiring javascript.
It could be an option whether the output shall still be in a code block.

A specific use case for this is plantuml (#3902). So with below code block everything from (including) @startuml to @enduml would be sent to the script that can generate a html code with an inline image of the uml.

```plantuml
@startuml
Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response

Alice -> Bob: Another authentication Request
Alice <-- Bob: Another authentication Response
@enduml
````

[zeripath]

So goldmark has a chroma extension which behaves exactly like that

[silverwind]

Related: #7729

[zeripath]

So the way to do this might be to add js that detects a language class and then sends it back to the server or even just renders it á la Mermaid.js

[davidsvantesson]

Should be possible to do something similar for plantuml (with a plantuml server). Although the code is not fully completed, it seem to be only minor adaptions required:
https://plantuml.com/code-javascript-synchronous

I don't have time right now, but I will try to do something with it later.

[silverwind]

detects a language class and then sends it back to the server

I'd advice against it. We should just use server-side highlighting. highlight.js detection is very inaccurate and it would essentially be a useless round trip if we can already detect it server-side.

[davidsvantesson]

Hmm... ok let's discuss in #9894. 😕

[davidsvantesson]

We have two examples for javascript based rendering now (PlantUML and mermaid). Server-side external rendering is only relevant if #7729 is implemented. Maybe it is enough to track in that one and close this one for now?

[lunny]

So goldmark has a chroma extension which behaves exactly like that

Maybe we could enable it now since #12047 merged.