feat(inspector): start inspector server on SIGUSR1 (#35738)
In Node.js you can start the inspector on an already running process by
sending it SIGUSR1, which is useful for debugging long-running tasks
that cannot easily be restarted with --inspect. This adds the same capability
to deno: on Unix, sending SIGUSR1 starts the inspector server on the
default 127.0.0.1:9229 without pausing execution, and prints the usual
"Debugger listening on ws://..." banner.
The listener is installed after a 500ms grace period so short-lived
programs don't pay the cost of a process-wide signal handler (a trivial
`deno eval` still exits in ~30ms). If the inspector server is already
running (an --inspect* flag, node:inspector's open(), or a previous
SIGUSR1), the signal is a no-op. The activation reuses the runtime
registration path that already backs node:inspector's open(). JS
listeners added with Deno.addSignalListener("SIGUSR1") keep working
alongside it.
Closes #9312
