Allow to export values with invalid OCaml names

[jchavarri]

At the moment, all values exported from the structure items of a module need to comply with OCaml ident spec:

ident ::= (letter ∣ _) { letter ∣ 0…9 ∣ _ ∣ ' } 

But sometimes, it is useful to export ids with names starting with uppercase (like for NextJS routes) or starting with numbers, etc.

In ReScript, this was solved at the parsing level allowing for illegal identifiers (example). A solution like this is probably not possible in Melange, as compatibility with Reason and OCaml parsers (or at least the latter) is required, but maybe a solution with PPX attributes is possible?

[anmonteiro]

This is very much desirable. I believe one of the options we chatted offline was to use @mel.as, e.g.:

let[@mel.as POST] post a b = a + 1