fix(ext/node): handle signal 0 in child_process.kill() (#32294)
## Summary
- `child_process.kill(0)` threw `Unknown signal: 0` because
`toDenoSignal()` didn't recognize signal 0
- In POSIX, signal 0 checks if a process exists without sending a signal
(`kill(pid, 0)`)
- Now handles signal `0` (both numeric and string `"0"`) as a special
case in `ChildProcess.kill()`, using `process.kill(pid, 0)` which
already supports signal 0
- This broke npm packages like `nuxt` that use `kill(0)` to check if a
child process is alive on exit
Closes #25776
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
