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:

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.

Start with the child_process subprocess.kill API and compare the Unix process-group approach with Windows taskkill /T /F described in the issue. Define the cross-platform killTree semantics, signal mapping, and documented limitations, then verify behavior on Windows, Linux, and macOS.