Provide more flexibility with the navigation block

Attached patch

Okay I think I've got what you are trying to achieve. I've refactored a bit to make the alter of the menu item broader to allow the full render array to be modified.

You should now be able to do this:

/**
 * Perform alterations on the chapter link item markup.
 *
 * @param array $chapter_menu_item
 *   The chapter menu item including link and icon.
 */
function MODULE_NAME_epub_reader_framework_navigation_chapter_menu_item_alter(&$chapter_menu_item) {
  $chapter_menu_item['#attributes'] = array_merge_recursive([
    'class' => ['u-container'],
  ]);
}

I've also added the `chapter_label` and `chapter_heading` alters to alter the two labels. I've kept `chapter_heading` as the headings within the chapter as that matches the object name and database storage for it.