After launching an app, if successful, we connect stdio streams to it,
and wait for it to exit before we return to the prompt.
stdio is implemented as regular AppConnection messages:
- stdin: `{ $: 'stdin', data: Uint8Array }` from phoenix -> child
- stdout: `{ $: 'stdout', data: Uint8Array }` from child -> phoenix
Terminal and Phoenix now communicate with each other using the same
style, instead of 'input' and 'output' messages. This will help with
eventually running subshells.
SIGINT currently is not sent. We also suffer from the same "one more
read from stdin happens after app exits" bug that's in
PathCommandProvider where I copied the stdin code from.