allow dead code for StyleEditionDefault

We need to allow `StyleEditionDefault` because it will be used to
implement `style_edition`, but that work is currently ongoing.

src/config/style_edition.rs

@@ -1,6 +1,7 @@
 use crate::config::StyleEdition;
 
 /// Defines the default value for the given style edition
+#[allow(dead_code)]
 pub(crate) trait StyleEditionDefault {
     type ConfigType;
     fn style_edition_default(style_edition: StyleEdition) -> Self::ConfigType;