Spawn child processes directly and drop the Deno.Command shim

src/errors.ts

@@ -1,5 +1,4 @@
-// Node reports a missing file as ENOENT and a missing process as ESRCH, where
-// Deno threw `Deno.errors.NotFound` for both.
+// A missing file or a missing process: Node reports them as ENOENT and ESRCH.
 export function isNotFound(error: unknown): boolean {
   if (!(error instanceof Error) || !("code" in error)) {
     return false;