Support syn::Pat::Wild pattern in method signatures

[girstenbrei]

This issue relevant after #20 merges, but is not a result of the PR.

Assuming #20, the following fails to compile:

enum Abort {
    Yes,
    No,
}

#[trait_variant::make(Example: Send)]
trait LocalExample {
    async fn should_abort(_: String) -> Abort {
        Abort::No
    }
}

The reason is the unsupported pattern _, tracing back to this error:

impl-trait-utils/trait-variant/src/variant.rs

Line 199 in 6a5e7ab

_ => Error::new_spanned(pat, "patterns are not supported in arguments")

Giving the variable a name is possible, of course, but leads to the slight inconvenience of requiring allow(clippy::no_effect_underscore_binding) and allow(clippy::used_underscore_binding).