Support Web Assembly

One of the cool things about `nusb` (and rust, for that matter), is *trivial* targeting of web-assmebly. With WebUSB, this would mean we could write a browser-native scanner GUI for the USB scanners, allowing users a nice GUI experience, without needing to download or install anything.

Of course this is chromium only (I get why mozilla doesn't like it, but it is still frustrating).

Sadly, this would be USB-only of course also.

However, if we designed the frontend clever enough, we could have a nkscan --gui option to basically host a webserver and proxy the connection, which would support non-chromium and firewire.
This of course would not require webassembly.