fix(spawn): use detached:true so children are immune to terminal SIGINT

Children now get their own process group. Terminal Ctrl+C only reaches
the parent, which then does orderly tree-kill while children are still
alive and the process tree is intact.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "@push.rocks/smartshell",
   "private": false,
-  "version": "3.3.4",
+  "version": "3.3.5",
   "description": "A library for executing shell commands using promises.",
   "main": "dist_ts/index.js",
   "typings": "dist_ts/index.d.ts",