Support configuration extension patterns

When designing configuration systems, provide both replacement and extension options for list-based settings to give users flexibility in how they modify default configurations.

For list-based configurations, consider implementing:

1. A base option for complete replacement (allow_abc_meta_bases)
2. An extension option that appends to defaults (extend_abc_meta_bases)

This approach allows users to either:

• Override the entire list when they need complete control
• Add to the existing defaults when they only need minor adjustments

#[option(
    default = r#"["typing.Protocol", "typing_extensions.Protocol"]"#,
    value_type = "list[str]",
    example = r#"allow-abc-meta-bases = ["my_package.SpecialBaseClass"]"#
)]
pub allow_abc_meta_bases: Vec<String>,

#[option(
    default = r#"[]"#,
    value_type = "list[str]", 
    example = r#"extend-abc-meta-bases = ["my_package.SpecialBaseClass"]"#
)]
pub extend_abc_meta_bases: Vec<String>,

When implementing the options, combine them at the settings level rather than requiring consumers to check both settings:

// In your settings structure
pub struct Settings {
    // Combined at initialization time
    pub allowed_bases: FxHashSet<String>,
}

// When constructing settings
let mut allowed_bases = options.allow_abc_meta_bases.into_iter().collect::<FxHashSet<_>>();
allowed_bases.extend(options.extend_abc_meta_bases.iter().cloned());

This pattern promotes cleaner code that consumes the settings, making it less error-prone and easier to maintain.