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/normal.c b/src/normal.c
index 6620af9..8a4e778 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -1386,6 +1386,7 @@
 #endif
 
     VIsual_active = FALSE;
+    trigger_modechanged();
     setmouse();
     mouse_dragging = 0;
 
@@ -5642,6 +5643,7 @@
 	{				    //	   or char/line mode
 	    VIsual_mode = cap->cmdchar;
 	    showmode();
+	    trigger_modechanged();
 	}
 	redraw_curbuf_later(INVERTED);	    // update the inversion
     }
@@ -5757,6 +5759,7 @@
     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.