Function tidy::unstable_book::collect_unstable_book_lib_features_section_file_names

fn collect_unstable_book_lib_features_section_file_names(
    base_src_path: &Path,
) -> BTreeSet<String>

Retrieves file names of all language feature sections in the Unstable Book with:

• hyphens replaced by underscores,
• the markdown suffix (‘.md’) removed.