Add support for yes/no booleans

[ikskuh]

Some programs require "troolean" options that can have a default, but also be forced to true and false.

We can either implement it as a special case for ?bool or provide a custom type for this:

const Trinary = enum { unset, false, true };

var cli = parse(struct {
    @"emit-bin": ?bool = null, // --emit-bin and --no-emit-bin
    @"emit-bin": args.Trinary = .unset, // --emit-bin and --no-emit-bin
}, …);