completion for \<^const>, although it often requires an extra argument;

--- a/etc/symbols	Sun Apr 28 12:34:56 2019 +0200
+++ b/etc/symbols	Sun Apr 28 13:03:16 2019 +0200
@@ -394,6 +394,7 @@
 \<^class>               argument: cartouche
 \<^class_syntax>        argument: cartouche
 \<^command_keyword>     argument: cartouche
+\<^const>               argument: cartouche
 \<^const_abbrev>        argument: cartouche
 \<^const_name>          argument: cartouche
 \<^const_syntax>        argument: cartouche