Add return value to `emscripten_proxy_async_with_callback`

[sbc100]

For my use case in #18376 it would be useful to have a void* return value for the function being proxied. I would like to be able to then use the in my callback.

[sbc100]

I can work around this by defining a new "pair" struct and stashing the result in that.. but its less elegant.

[tlively]

Would you want the new callback argument to replace the current callback_arg chosen by the initial caller or to be passed alongside it?

IMO it would make sense to pass both, but then there is the usability downside of having two void* arguments that can be confused.