patch 8.2.4713: plugins cannot track text scrolling

Problem:    Plugins cannot track text scrolling.
Solution:   Add the WinScrolled event. (closes #10102)
diff --git a/src/edit.c b/src/edit.c
index f30edd5..1585f85 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -1527,6 +1527,9 @@
 					(linenr_T)(curwin->w_cursor.lnum + 1));
     }
 
+    if (ready)
+	may_trigger_winscrolled(curwin);
+
     // Trigger SafeState if nothing is pending.
     may_trigger_safestate(ready
 	    && !ins_compl_active()