Don't wait for a child that has already exited to quit

src/Supervisor.ts

@@ -19,8 +19,12 @@ export class Supervisor extends EventEmitter {
    * See https://azimi.me/2014/12/31/kill-child_process-node-js.html for more information
    */
   async stop() {
+    // if we never started the child process, we don't need to do anything
     if (!this.process || !this.process.pid) return;
 
+    // if the child process has already exited, we don't need to do anything
+    if (this.process.exitCode !== null) return;
+
     const ref = this.process;
     const exit = once(ref, "exit");
     this.kill("SIGTERM");