squash WCP variables and scopes

As discussed, WCP add_variables and add_scope are problematic because this means that all WCP clients and servers need to agree on what is a variable and what is a scope. Instead, this MR implements add_items which removes this distinction and asks the server to add whatever it can find which matches the provided name(s). The recursive flag remains but is obviously ignored if the name is found to be a variable.

Leaving the old commands in for now. I'll post another MR later once we've updated the clients. I already have a slang-server commit doing this and don't feel we need a very long deprecation time since that project just launched and anyone who is using it should be comfortable keeping things head-to-head for now. @lklemmer can advise on what I believe is the only other extant client.