patch 8.2.3591: no event is triggered when closing a window

Problem:    No event is triggered when closing a window.
Solution:   Add the WinClosed event. (Naohiro Ono, closes #9110)

src/vim.h

@@ -1379,6 +1379,7 @@ enum auto_event
     EVENT_WINENTER,		// after entering a window
     EVENT_WINLEAVE,		// before leaving a window
     EVENT_WINNEW,		// when entering a new window
+    EVENT_WINCLOSED,		// after closing a window
     EVENT_VIMSUSPEND,		// before Vim is suspended
     EVENT_VIMRESUME,		// after Vim is resumed