Add a #[kwarg(with_syn)] attribute

Added

• Support a #[kwarg(with_syn)] shorthand to parse via syn::Parse instead of MacroArg (smwonvvz)
  • More concise than using the proc_macro_kwargs::parse::Syn wrapper type (which doesn't even support #[kwarg(with_wrapper)])
  • Also available through the #[kwarg(syn)] shorthand