patch 8.2.4911: the mode #defines are not clearly named

Problem:    The mode #defines are not clearly named.
Solution:   Prepend MODE_.  Renumber them to put the mapped modes first.
diff --git a/src/option.c b/src/option.c
index 518e787..92b1b93 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2915,7 +2915,7 @@
     {
 	if (p_im)
 	{
-	    if ((State & INSERT) == 0)
+	    if ((State & MODE_INSERT) == 0)
 		need_start_insertmode = TRUE;
 	    stop_insert_mode = FALSE;
 	}
@@ -3110,7 +3110,7 @@
 	// Only de-activate it here, it will be enabled when changing mode.
 	if (p_imdisable)
 	    im_set_active(FALSE);
-	else if (State & INSERT)
+	else if (State & MODE_INSERT)
 	    // When the option is set from an autocommand, it may need to take
 	    // effect right away.
 	    im_set_active(curbuf->b_p_iminsert == B_IMODE_IM);