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_getln.c b/src/ex_getln.c
index ad0f07e..a49fa05 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -1696,6 +1696,10 @@
     // Trigger CmdlineEnter autocommands.
     cmdline_type = firstc == NUL ? '-' : firstc;
     trigger_cmd_autocmd(cmdline_type, EVENT_CMDLINEENTER);
+#ifdef FEAT_EVAL
+    if (!debug_mode)
+	trigger_modechanged();
+#endif
 
     init_history();
     hiscnt = get_hislen();	// set hiscnt to impossible history value
@@ -2461,6 +2465,12 @@
     trigger_cmd_autocmd(cmdline_type, EVENT_CMDLINELEAVE);
 
     State = save_State;
+
+#ifdef FEAT_EVAL
+    if (!debug_mode)
+	trigger_modechanged();
+#endif
+
 #ifdef HAVE_INPUT_METHOD
     if (b_im_ptr != NULL && *b_im_ptr != B_IMODE_LMAP)
 	im_save_status(b_im_ptr);