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/gui.c b/src/gui.c
index 68ac9d8..23694d1 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5237,6 +5237,9 @@
 	last_cursormoved = curwin->w_cursor;
     }
 
+    if (!finish_op)
+	may_trigger_winscrolled(curwin);
+
 # ifdef FEAT_CONCEAL
     if (conceal_update_lines
 	    && (conceal_old_cursor_line != conceal_new_cursor_line