feat(meta): add Neg variant to Arg for negated flags

[ldseraph]

Add support for negated flags (e.g. -debug, -clone) to disable bool fields in #[derive(Attribute)] structs.

Changes:

• Add Arg::Neg(Ident) variant with parsing, ToTokens, is_neg(), as_neg()
• Add Args::has_flag() and Args::has_neg() for variant-aware queries
• FromArg for bool: Neg returns false (Flag returns true)
• FromArg for Ident/Path: accept both Flag and Neg
• emit.rs: bool fields respect #[zyn(default = true/false)]
  • default = true → !args.has_neg(key) (on unless negated)
  • default = false → args.has_flag(key) (same as no default)
  • non-literal expr → generic match + FromArg fallback

Usage:
#[derive(Attribute)]
#[zyn("opt")]
struct Opt {
#[zyn(default = true)]
debug: bool, // on by default, use -debug to disable
display: bool, // off by default, use display to enable
}