patch 8.2.4713: plugins cannot track text scrolling

Problem:    Plugins cannot track text scrolling.
Solution:   Add the WinScrolled event. (closes #10102)

src/vim.h

@@ -1386,6 +1386,7 @@ enum auto_event
     EVENT_WINCLOSED,		// after closing a window
     EVENT_VIMSUSPEND,		// before Vim is suspended
     EVENT_VIMRESUME,		// after Vim is resumed
+    EVENT_WINSCROLLED,		// after Vim window was scrolled
 
     NUM_EVENTS			// MUST be the last one
 };