patch 8.2.3430: no generic way to trigger an autocommand on mode change

Problem:    No generic way to trigger an autocommand on mode change.
Solution:   Add the ModeChanged autocommand event. (Magnus Gross, closes #8856)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 06bff80..617de6f 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -485,6 +485,7 @@
     else
 	exmode_active = EXMODE_NORMAL;
     State = NORMAL;
+    trigger_modechanged();
 
     // When using ":global /pat/ visual" and then "Q" we return to continue
     // the :global command.