Add logic to sort posts by hierarchy

[oandregal]

Trac ticket https://core.trac.wordpress.org/ticket/62701

See related Gutenberg issues/PRs:

• Issue Showcase hierarchy in table/list layout gutenberg#55957
• PR Proof of concept: visualize hierarchical data gutenberg#66479

What

This PR expands the post controller to allow returning the data by parent-child relationship (sort by hierarchy).

Why

We want to display data in a hierarchical way in the site editor, similarly to what we do in wp-admin:

wp-admin	site editor

How to test

Added a new test suite to test the logic. Without the related Gutenberg PR this is difficult to test.

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