child_process: add cross-platform subprocess.killTree([signal])

### What is the problem this feature will solve? `subprocess.kill([signal])` only targets the direct child PID. Descendants often keep running, especially when: - `shell: true` (shell exits, real command survives) - the child is a build driver (`bash` / `make` / `ninja`) that spawns grandchildren - on Windows, where POSIX signals are not real and `kill()` does not terminate a process tree Userland workarounds are fragmented and OS-specific: - Unix: `detached: true` + `process.kill(-pid, signal)` (process group; not always applicable) - Windows: `taskkill /pid <pid> /T /F` (or equivalent) - npm packages such as `tree-kill` This comes up often for Ctrl+C / timeout cleanup in CLIs and build orchestrators. Related prior request (closed stale): https://github.com/nodejs/node/issues/40438 (`killDeep`). ### What is the feature you are proposing to solve the problem? Add an explicit, documented API, for example: ```js subprocess.killTree([signal]); // and/or child_process.killTree(pid[, signal]); ``` Goals: 1. **Cross-platform**: one call that best-effort terminates the process and its descendants on Windows, Linux, and macOS. 2. **Clear semantics** vs `kill()`: `kill()` stays single-PID; `killTree()` is the tree variant. 3. **Document limitations**: PID reuse races, process-group vs true tree walk, signal mapping on Windows (where signals are not POSIX). A reasonable Windows implementation can use the platform tree-terminate facility (today: `taskkill /T`); Unix can use process-group kill when the child is a group leader, otherwise a documented tree walk. ### What alternatives have you considered? - Keep using `tree-kill` / ad-hoc `taskkill` in every project - Rely on `detached` + negative PID (Unix-only, requires spawn options up front) - Expand `subprocess.kill()` to kill trees (breaking / surprising; prefer a separate `killTree` name) Naming `killTree` (rather than overloading `kill`) makes the stronger, OS-dependent behavior opt-in and discoverable.