feat: Compatible compilation to other WebAssembly environments

[cbbfcd]

Troubleshooting a problem where WebAssembly compiles into a browser or Emscripten environment with an error that the is_valid implementation is not found.

[Xaeroxe]

I'm curious what you expect which to do here. wasi presents a filesystem, the target you've specified here, as far as I'm aware, doesn't have a filesystem. I could merge this and it would probably build, but then I suspect you'd have a runtime failure.

[cbbfcd]

Thanks for the reply.
Because of the indirect dependency on which, which then needs to be compiled into WebAssembly to run on the browser side, there is really no filesystem, but it is guaranteed to compile without reporting errors.

[Xaeroxe]

Okay, perhaps it would be better to make which an optional feature in your dependency tree. I'm not sure how it enters your dependency graph, but can you go to the dependency that brings which in and instead make which optional?

[cbbfcd]

I've tried, but the reference hierarchy is a bit deep. Now, I also feel that this PR is not very reasonable, although adding this code doesn't seem to affect which.

[Xaeroxe]

The thing that makes me a bit uncomfortable with it is that by adding this code we're implying that which could do something worthwhile in that environment, when that's just not true. I don't particularly want to knowingly ship broken code. I think a compile failure is preferable to an unexpected runtime failure. Please either target wasm32-wasi, or find a way to remove which from your dependency graph.