[Build] Investigate Melange as an alternative to Js_of_ocaml

[WardBrian]

Melange (https://github.com/melange-re/melange) recently hit 1.0 and (seemingly) offers similar features to Js_of_ocaml: https://melange.re/v1.0.0/melange-for-x-developers/#for-js_of_ocaml-developers

It would be interesting to build a version of stancjs with this tool instead (probably requiring some source edits, at a minimum a bump to OCaml 4.14). The main thing I am interested in is if the produced Javascript is faster than the existing (which would help RStan, see: https://github.com/stan-dev/rstan/pull/1087#issuecomment-1711751769)