Add support for unit closer pages in toc

[TylerZeroMaster]

https://openstax.atlassian.net/browse/CORE-930

The default for unit pages will remain the same: treat them as introductions

This new unit-closer class supports specifying pages that should appear at the end of the unit.

I debated on if this should be done structurally instead, like this:

      # if this page is inside a unit that has at least one chapter and there are no chapters
      # after this page
      @is_unit_closer ||= has_ancestor?(:unit) and
        ancestor(:unit).element_children.only(ChapterElement).any? and
        ancestor(:unit)
          .element_children
          .drop_while { |el| el[:id] != self[:id] }
          .none? { |el| el[:'data-type'] == 'chapter' }

I decided not to do this since introductions are done with classes and it could be useful to select unit closers by a css selector. I am a little concerned that using a class could cause confusing behavior if a unit-closer is placed before the end of a unit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (abe1663) to head (456499f).

@@            Coverage Diff            @@
##              main      #408   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          217       217           
  Lines         3583      3588    +5     
=========================================
+ Hits          3583      3588    +5     

Files with missing lines	Coverage Δ
lib/kitchen/directions/bake_toc.rb	100.00% <100.00%> (ø)
lib/kitchen/page_element.rb	100.00% <100.00%> (ø)

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[jivey]

I don't have a deep understanding of the cookbook but this appears like a solid approach.