Parse list arguments to built-in commands as list nodes

[nmoroze]

There are certain built-in commands that interpret arguments as lists. Parsing these as List nodes would enable us to do some more sophisticated checks. In some cases, we can do argument count checks. In addition, we could report spacing violations between list items.

If we do this for proc nodes, we could also add a checker for correct arg count to procs. It wouldn't catch everything until we can follow source statements, but it'd still catch additional bugs.