Tabs component

[peteryates]

Add a tabs component using the new Slots API. The ViewComponent Slot API is the experimental replacement for
ContentArea. It offers far more power and lets us repeat (via collection) calls to slots, so we can define an unlimited number of tabs with a clean DSL-like syntax.

The main bonus here, and the cause of the delay on this work, is that now testing block functionality is fully supported by render_inline taking a block.

render GovukComponent::Tabs.new(title: 'Days of the week') do |component|
  component.slot(:tab, title: 'Monday') do
    tag.p("Monday's child is fair of face", class: 'govuk-body')
  end

  component.slot(:tab, title: 'Tuesday') do
    tag.p("Tuesday's child is full of grace", class: 'govuk-body')
  end

  component.slot(:tab, title: 'Wednesday') do
    tag.p("Wednesday's child is full of woe", class: 'govuk-body')
  end
end