Feature enhancement: Provide Twig filter extension to support object/node dumping

I've tried to make some changes to the clean template, but struggle to find any documentation on what fields are available. Therefore could a filter be added (say get_items) which would work in the following scenario:

  {% for key,value in object | get_items %}
          <p> {{key}}  : {{object}} </p>
  {% endfor %}

The advantage of this approach is it would provide a mechanism for template developers/tweekers to understand the 'art of the possible' avoiding the need to document every object.

Start by locating the clean template and the existing Twig filter extension points. Confirm how template objects and nodes are exposed, then define the filter's expected iteration output and add coverage for the example in the issue; done means template developers can inspect available fields without manually documenting every object.