Fix ignored variant requires Default trait

[rezigned]

Ignored variants like the following code shouldn't require a Default trait implementation.

The Default trait was introduced in v1.0.0-rc3, which allows users to capture default values into Named and Unnamed variants. However, it doesn't respect the ignore field. Thus, any code that uses the ignore field requires the Default trait to be implemented (undesired behavior).

#[derive(Debug, PartialEq, Eq, keymap_derive::KeyMap, Clone)]
enum Foo {
    /// Active variant
    #[key("a")]
    Bar,
    /// Ignored variant (should NOT require `Default` trait)
    #[key(ignore)]
    Baz(Baz),
}