Syntax for metadata on code blocks

[Julow]

It should be possible to attach metadata to code blocks. There is two main use-cases:

• Running Mdx on OCaml and mld files. It needs per-block configuration.
• Code highlighting. The HTML backend adds color highlighting but assumes that all blocks contain OCaml code.

Odoc won't try to interpret or to render them, they would be present in the parse tree but discared afterwards.

During a previous meeting, we proposed the following syntax:

{@language metadata [
]}

The metadata part is an arbitrary string and is optional.
We'd need quoting, I propose wrapping it with double quotes ("") and allowing OCaml's escaping (eg. \" and everything String.escape produces).

The language part is not optional.

For example:

{@sh [
rm -rf /
]}

{@ocaml env=f1 version>=4.06 [
# let x = 1 ;;
val x : 1
]}

[dbuenzli]

Code extraction is another use case. There's a long discussion about it in #303

[jonludlam]

@dbuenzli yes, I'm hoping mdx will do that job for us. It's got code extraction working already (but from markdown files).

[dbuenzli]

Honestly I rather not add more tools to the basics I need for dev (and especially not tools that do black magic). It doesn't seem super hard to add file extraction to odoc itself by starting from the document IR.

[Julow]

This is implemented in https://github.com/ocaml-doc/odoc-parser and has been released (1.0.0).