Work item Hierarchy widget - Empty state

Related to #362852 (closed)

Creates base component for work item children with empty state and form. This does not include any API queries.

Behind :work_items feature flag.

This first iteration does not include the tree icon as per the design as it's not available in our SVG library yet: #343769 (comment 953769602)

Screenshots

How to test

1. Enable work_items feature flag

   Feature.enable(:work_items)

2. Visit an issue page, and add a checklist item to description

3. Hover over checklist item, then click 'Convert to task' button on the left

4. Save the work item

5. Click on the newly created work item

6. Click right chevron on child items block

7. Click Add a child button

8. The form currently does not do anything