Add docs for import_module attribute

[SoniEx2]

Currently emscripten does not document how to import from non-env modules. It would be very helpful if it did.

[sbc100]

We don't currently have any support for that. All imports come from env (or in some rare cases wasi_snapshot_preview1), but there is not support for extending this today.

[SoniEx2]

you support standalone wasm, but standalone wasm is unusable without custom imports

[sbc100]

Oh in standalone wasm mode you can actually import whatever you want using LLVM attributes: __attribute__((import_module("mymodule"), import_name("myfunc"))) void func;

[SoniEx2]

yeah, it'd be helpful if that were somewhere in the emscripten docs (maybe alongside standalone wasm?). we did eventually figure out to look at llvm docs for it but it took us longer than we'd like to admit.

[bnason-nf]

Related to this, it would be nice if js-library or port implementations could populate items to other import module names besides "env".

[sbc100]

I agree this feature would be great. I've tried to implement it few times now without success, becuase it seems to add a lot of complexity. But I'm hopefully we can find a way to do it one day...