Support running servers on wasm

[johanhelsing]

Hi :)

I was looking at this project and https://github.com/ErnWong/dango-tribute and it seems like @ErnWong hacked together a version that lets the server be run in the browser (in its own iframe, I think).

I think most of it is in this commit: ErnWong/dango-tribute@f1f6942

Maybe it could be added as a feature flag instead of being #[cfg(not(target_arch = "wasm32"))]?

The reasons I'm interested in this are:

1. Reduce latency if the host is close to the other players, for instance if they're on the same local network.
2. Lower cost, I just have to host a signaling/matchmaking server. At least as long as I'm not worried about cheating.