patch 8.1.2348: :const cannot be followed by "| endif"

Problem:    :const cannot be followed by "| endif".
Solution:   Check following command for :const. (closes #5269)
            Also fix completion after :const.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 385cf3b..576a756 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2359,6 +2359,7 @@
 	    case CMD_browse:
 	    case CMD_call:
 	    case CMD_confirm:
+	    case CMD_const:
 	    case CMD_delfunction:
 	    case CMD_djump:
 	    case CMD_dlist: