fix: emit original token stream when there are syntax errors in #[component] or #[island] function signature (closes #2133)

[gbj]

There's a compile-time trade-off here, as this requires cloning the TokenStream another time so makes every #[component] macro slightly slower in exchange for correctly passing along errors when the function signature is broken.

[pheki]

I think that the extra clone can be removed by checking the dummy result earlier:

    let Ok(mut dummy) = syn::parse::<DummyModel>(s.clone()) else {
        return s.into();
    };
    let parse_result = syn::parse::<component::Model>(s);

    if let (ref mut unexpanded, Ok(model)) = (&mut dummy, parse_result) {
        let expanded = model.is_transparent(is_transparent).into_token_stream();
        unexpanded.sig.ident =
            unmodified_fn_name_from_fn_name(&unexpanded.sig.ident);
        quote! {
            #expanded
            #[doc(hidden)]
            #[allow(non_snake_case, dead_code, clippy::too_many_arguments)]
            #unexpanded
        }
    } else {
        dummy.sig.ident = unmodified_fn_name_from_fn_name(&dummy.sig.ident);
        quote! {
            #[doc(hidden)]
            #[allow(non_snake_case, dead_code, clippy::too_many_arguments)]
            #dummy
        }
    }
    .into()

[gbj]

Oh GREAT point. Yeah this is perfect. Fixes both #2133 and (still fixes) #1781, no additional cost. Thanks!