Adds support for running the tunnel as a service on macOS via launchservices.
It also hooks up observability (`code tunnel service log`) on macOS and Linux.
On macOS--and later Windows, hence the manual implementation of `tail`--it
saves output to a log file and watches it. On Linux, it simply delegates to
journalctl.
The "tailing" is implemented via simple polling of the file size. I didn't want
to pull in a giant set of dependencies for inotify/kqueue/etc just for this use
case; performance when polling a single log file is not a huge concern.
Connor Peet