当子进程的 stdio 流被关闭时触发。
'exit' 事件的区别是，多个进程可能共享同一 stdio 流。
code<number> The exit code if the child exited on its own.
signal<string> The signal by which the child process was terminated.
'close' event is emitted when the stdio streams of a child process have
been closed. This is distinct from the
'exit' event, since multiple
processes might share the same stdio streams.