[FEATURE REQUEST] file system support

We use the `--file=foo` command line arg in js_of_ocaml quite heavily for attaching metadata to bundles - see https://github.com/janestreet/core_kernel/blob/fc5f4d527cbdf0d3b2f13de7e316baf8f95b92ef/version_util/src/version_util.js#L29. It would be great to have this in wasm as well.