feat: ensure spawned child procs are killed

[xendarboh]

Improve management of spawned processes on all platforms. Generally works now and can use closer examination to be more robust for some edge cases and platform specifics.

• on exit
  • https://v2.tauri.app/reference/javascript/api/namespacewindow/#oncloserequested
  • Kill spawned processes on App close from Javascript Code tauri-apps/tauri#5870
• before (re)spawning a new command