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/proto/autocmd.pro b/src/proto/autocmd.pro
index 24dd1ba..d8145df 100644
--- a/src/proto/autocmd.pro
+++ b/src/proto/autocmd.pro
@@ -25,6 +25,7 @@
 int has_cmdundefined(void);
 int has_textyankpost(void);
 int has_completechanged(void);
+int has_modechanged(void);
 void block_autocmds(void);
 void unblock_autocmds(void);
 int is_autocmd_blocked(void);