Allow passing melange.module and bs.module payloads for assets

[jchavarri]

Considering that:

• the placement of assets like images or fonts might change while migrating to Dune
• some apps are large enough that they migrate some parts to Melange progressively, not in a one-shot commit

it would be nice to support passing two payloads for assets, e.g.:

external img : string = "default"
  [@@bs.module "../../public/assets/images/robot.svg"]
  [@@melange.module "../../../../../robot.svg"]

This would allow to proceed with progressive migrations without having to turn to conditional compilation. It would also set Melange up to progressively migrate to melange.X attributes over bs.* ones.

[jchavarri]

I don't think this is needed anymore, as there is now support to copy any assets from any folder:

• from melange.emit stanzas: feat(melange): add runtime_deps to copy assets to the target dir ocaml/dune#7199
• from library stanzas: feat(melange): add melange.runtime_deps field in library ocaml/dune#7234