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/digraph.c b/src/digraph.c
index 9fdd0b3..cfffc19 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2566,7 +2566,7 @@
 	vim_snprintf((char *)buf, sizeof(buf), "<buffer> %s %s",
 				((kmap_T *)curbuf->b_kmap_ga.ga_data)[i].from,
 				 ((kmap_T *)curbuf->b_kmap_ga.ga_data)[i].to);
-	(void)do_map(2, buf, LANGMAP, FALSE);
+	(void)do_map(2, buf, MODE_LANGMAP, FALSE);
     }
 
     p_cpo = save_cpo;
@@ -2597,7 +2597,7 @@
     for (i = 0; i < curbuf->b_kmap_ga.ga_len; ++i)
     {
 	vim_snprintf((char *)buf, sizeof(buf), "<buffer> %s", kp[i].from);
-	(void)do_map(1, buf, LANGMAP, FALSE);
+	(void)do_map(1, buf, MODE_LANGMAP, FALSE);
     }
     keymap_clear(&curbuf->b_kmap_ga);