vim-patch:8.2.4723: the ModeChanged autocmd event is inefficient

Problem:    The ModeChanged autocmd event is inefficient.
Solution:   Avoid allocating memory. (closes vim/vim#10134)  Rename
            trigger_modechanged() to may_trigger_modechanged().
2bf52dd065

Make v:event readonly for ModeChanged.

src/nvim/ex_getln.c

@@ -921,7 +921,7 @@ static uint8_t *command_line_enter(int firstc, long count, int indent, bool init
     }
     tl_ret = true;
   }
-  trigger_modechanged();
+  may_trigger_modechanged();
 
   state_enter(&s->state);
 
@@ -6556,7 +6556,7 @@ static int open_cmdwin(void)
   cmdmsg_rl = save_cmdmsg_rl;
 
   State = save_State;
-  trigger_modechanged();
+  may_trigger_modechanged();
   setmouse();
 
   return cmdwin_result;