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/normal.c

@@ -3050,6 +3050,7 @@ static int get_mouse_class(char_u *p)
 void end_visual_mode(void)
 {
   VIsual_active = false;
+  trigger_modechanged();
   setmouse();
   mouse_dragging = 0;
 
@@ -6680,6 +6681,7 @@ static void nv_visual(cmdarg_T *cap)
                                               //           or char/line mode
       VIsual_mode = cap->cmdchar;
       showmode();
+      trigger_modechanged();
     }
     redraw_curbuf_later(INVERTED);          // update the inversion
   } else {                // start Visual mode
@@ -6782,6 +6784,7 @@ static void n_start_visual_mode(int c)
   VIsual_mode = c;
   VIsual_active = true;
   VIsual_reselect = true;
+  trigger_modechanged();
   // Corner case: the 0 position in a tab may change when going into
   // virtualedit.  Recalculate curwin->w_cursor to avoid bad highlighting.
   //