`sidetone`: Create a service

[jbrubake]

See https://www.bentasker.co.uk/posts/documentation/linux/adding-mic-monitoring-to-headphone-output-on-linux.html. You could use sidetone by itself or use it to control the service.

There should be a way to "lock" sidetone so the service keeps running but doesn't change the state of the sidetone.

This could be done with options or subcommands to sidetone. Options would allow lock/unlock while also start/stop so is probably better? Or you could do both so that you could lock/unlock without start/stop?

Maybe put all functionality in the service and use signals to change things? Really need to think through the interface here.