[herd-www] Add .opam file specifying dependencies

[TiberiuBucur]

This PR addresses a mismatch between zarith versions >=1.13, required by herdtools7.opam, and zarith_stubs_js, required by herd-www. The oldest version of zarith_stubs_js that does not conflict with zarith 1.13 is v.0.16.1. This version requires Ocaml >=4.14.0.
This .opam file specifies these requirements, giving the user the ability to automatically install the right versions of the packages through opam install ($OPAM_FILE_PATH) --deps-only for herd-www, such that these will not conflict with the versions required by herdtools7.opam in the root of the repository.

[ShaleXIONG]

LGTM

[fsestini]

A couple of comments:

1. Let's update CI workflows to use this opam file to solve and install dependencies. Let's also update the README.md in herd-www to inform the reader of the possibility to do opam install . .. --deps-only from within herd-www/ to install dependencies. (I think opam install . --deps-only alone is not sufficient because herd-www depends on sources from the core herdtools package, which has further dependencies.)

2. Since herd-www builds against sources from the core herdtools package, it may be worth extending herd-www.opam to explicitly include the subset of dependencies from herdtools7.opam that are required to build herd-www. This would make herd-www.opam a more complete and declarative description of the dependencies needed to build herd-www, without relying indirectly on another opam file.

EDIT: I've double-checked in an empty opam switch and indeed just running opam install . --deps-only inside herd-www will miss zarith, which is required to build herdtools' lib/. Since it's a single dependency and we are already requiring zarith_stubs_js anyway, I think we should go with point (2) above and just add zarith to herd-www.opam.