import { SubCommand } from "https://deno.land/x/sane_fmt@0.14.0/x/args.ts";
Declare add subcommand to existing command parser
type
<Main extends CommandReturn<any, any, any>, Name extends string, Sub extends CommandReturn<any, any, any>, ErrList extends readonly ParseError[]>() => Command<SubCommandReturn<Main, Name, Sub>, ErrList>