Cleanly exit (child) process when receiving a signal

[unstubbable]

This is especially important when the talkback server is started as a child process, e.g. via tsx watch. Then the parent process will wait for the exit callback, see https://github.com/privatenumber/tsx/blob/50d81944b1a2ced81f0c4a575f18df340d56ec38/src/watch/index.ts#L122 for example.

[ijpiantanida]

Thank you @unstubbable