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.

Steps to test this manually:

(alternatively, you may want to generate a gutenberg zip from trunk, use the normal wordpress-develop setup and change the gutenberg's PHP code in the plugin directory)

• In this PR, execute npm install && npm run build:dev.
• Have a clone of the Gutenberg repository, switch to trunk, create a .wp-env.override.json with the following contents (core is the local path of this PR's repository, so change accordingly):

{
 "core": "../wordpress-develop/src"
}

• In the Gutenberg codebase, comment the 3 filters defined here.
• In the Gutenberg codebase, run npm run wp-env start. Then npm install && npm run dev.
• Visit localhost:8888, open the wp-admin, and go to "Tools > Import". Import this theme test data file, that comes with hierarchical pages (or create your own set of pages).
• Visit the Site Editor > Pages. Then switch to the table layout. You should see something like this:

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