[build] Generate `given_exo.{cmi,cmo,cmt}` files

[erikmd]

• Kind: feature

• Address #414 first (disabling broken option learn-ocaml build --jobs=2)

Description

• Add option learn-ocaml build --enable-cmo-build
• This new experimental option is on by default (implicit mode) and generates given_exo.{cmi,cmo,cmt} in the www folder for each prelude ^ " ;;\n" ^ prepare.
• This is a key feature for learn-ocaml.el (the Tuareg/Merlin frontend for Learn-OCaml); it can be disabled using the flag:

  $ learn-ocaml build --disable-cmo-build
  

• If the www/exo.json file is up-to-date, the exo.cm* are not built.
• But we can force the exo.cm* build (explicit mode) even for un-changed exercises using the flag:

  $ learn-ocaml build --enable-cmo-build
  

• This option may later become mandatory if/when the js_of_ocaml front-end also fetches similar *.cmo files.
• Regarding the ocamlc naming convention:
  require that the input exercise id basename exo-name matches [0-9A-Za-z_-]+, otherwise raise a warning,
  then bijectively replace _ → _0 and - → _1 in exo, then yield the output filename dir/given_exo_1name.cmo.

Checklist

• [x] Read the CONTRIBUTING.md guide.
• [x] Add/update tests
• [ ] Implement the learn-ocaml-client counterpart of this feature within this PR

Note to maintainers

• Read this wiki page
• Make sure the PR has a milestone
• Assign yourself before merging (using a regular Merge for this one)