vim-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 vim/vim#8856)
f1e8876fa2

N/A patches for version.c:

vim-patch:8.2.3434: function prototype for trigger_modechanged() is incomplete

Problem:    Function prototype for trigger_modechanged() is incomplete.
Solution:   Add "void".
28e591dd50

Fixes #4399.
Fixes #7416.

src/nvim/state.c

@@ -136,7 +136,7 @@ int get_real_state(void)
 /// @returns[allocated] mode string
 char *get_mode(void)
 {
-  char *buf = xcalloc(4, sizeof(char));
+  char *buf = xcalloc(MODE_MAX_LENGTH, sizeof(char));
 
   if (VIsual_active) {
     if (VIsual_select) {