Support killing process trees cross-platform - #1

Summary

• honor the existing tree option on macOS and Linux with pidtree, while preserving Windows taskkill behavior
• signal descendants leaf-first, tolerate children that exit during traversal, and protect the current process ancestry
• keep descendants and concrete name-matched root PIDs tracked through forceAfterTimeout and waitForExit, including children reparented after their root exits
• match Linux process titles through the command name as well as the executable basename, consistent with process-exists
• cover PID, process-name, opt-out, grandchildren, and reparented-child behavior; update the README and TypeScript definition

Resolves sindresorhus#21

This also addresses the maintainer feedback on the earlier attempt in #34: it uses pidtree instead of tree-kill, updates index.d.ts, and makes the default/tree-disabled regression tests cross-platform so the established Windows behavior is covered whenever the Windows job is enabled.

Validation

• npm test on Node.js 24: 29/29 tests, XO, and tsd pass
• npx --yes node@20 --test: 29/29 tests pass
• GitHub Actions passes the complete Node.js 20/24 × Ubuntu/macOS matrix: https://github.com/jamilahmadzai/fkill/actions/runs/30042371523
• five consecutive focused process-tree regression runs
• npm pack --dry-run
• git diff --check

This fork-local PR is the review artifact because upstream currently has pull requests disabled. It is ready to transplant or merge if upstream contributions are reopened.