Emit error when attempting to both forward and parse an attribute path

[TedDriggs]

Parsing and forwarding an attribute are mutually exclusive behaviors in darling-generated impls. Declaring the same attribute for both will always result in the attribute only being parsed.

Given the following code:

#[derive(FromDeriveInput)]
#[darling(attributes(example), forward_attrs(example, cfg))]
pub struct Receiver {
    attrs: Vec<syn::Attribute>
}

The placement of example in forward_attrs is erroneous. It would be useful if darling generated a compile-time error to flag the mistake.