feat: relaunch on SIGUSR2 (#4979)

This is because Node uses SIGUSR1 to enable the debug listener so even
if you just want to restart code-server you end up enabling the debug
listener as well.

Opted to leave the SIGUSR1 handler in to avoid breaking existing
workflows even though it does mean even if you only want to enable the
debug listener you will end up restarting code-server as well.  We could
consider removing it after a transition phase.