add divider to index_controls > list DSL

[pjmuller]

Feature

Thanks for introducing #3218
What would make it complete is that we can add dividers as well

list label: "Custom Index List", style: :primary, color: :slate, title: "A custom list" do
  link_to "Google", "https://google.com"
  divider # 👈
  link_to "Fish.com", "https://fish.com"
end

Current workarounds

None?