Improve signature infrastructure

The `config` command uses different kinds of named arguments, which
illustrates how it works.

src/parser.rs

@@ -6,7 +6,7 @@ crate mod registry;
 crate mod span;
 
 crate use ast::{ParsedCommand, Pipeline};
-crate use registry::{CommandConfig, CommandRegistry};
+crate use registry::{Args, CommandConfig, CommandRegistry};
 
 use crate::errors::ShellError;
 use lexer::Lexer;