move to the uproxy-lib v12 build world

Depends on:
https://github.com/uProxy/uproxy-churn/pull/41
`RtcToNet` uses it to detect when a user might be trying to connect to an address on the remote user's private network, e.g. `192.168.1.1`. This TypeScript definition was in uproxy-lib but there's no real reason for it to be since it's only used here:
https://github.com/uProxy/uproxy-lib/pull/78