Separate new tabmode in a setting

Return static tabmode as default,
separate previous change in `dynamic` mode.

Despite usefulness of the solution,
it was not approved to be a new default.

Vote (link below) amongst users to change default
has shown inconclusive result and it was not representative,
as it had low number of participants.
https://github.com/profanity-im/profanity/pull/1912#issuecomment-1816232546

src/command/cmd_ac.c

@@ -1068,6 +1068,7 @@ cmd_ac_init(void)
 
     statusbar_tabmode_ac = autocomplete_new();
     autocomplete_add(statusbar_tabmode_ac, "actlist");
+    autocomplete_add(statusbar_tabmode_ac, "dynamic");
     autocomplete_add(statusbar_tabmode_ac, "default");
 
     status_ac = autocomplete_new();