What problem does this address?

Currently, if you try to preview a non-hierarchical post, the Previewable interface does not return the post ID of the "parent" post.

What is your proposed solution?

There are a few options here:

• We could add a new field for the Previewable interface, exposing the "parent" post ID;
• We could add a new custom interface for non-hierarchical posts exposing the "parent" post ID;
• We could let API consumers to figure this out by themselves.

What alternatives have you considered?

Currently, API consumers need to circumvent this by getting the id in the params and swap for the one in the response in their apps.

But it is my understanding tools are not doing that and exposing a broken experience. (See the additional context below).

Additional Context

This is particularly helpful in a toolbar. See this Github issue, wpengine/faustjs#1515, for an example where this problem reflects.