What?

Refactor core/post-data block bindings source, and add unit test coverage. Modeled after #72739 and #73055.

This fixes a bug that was discovered by @jeryj, see #73406. For more information, see testing instructions below.

Why?

First of all, to fix a bug (see testing instructions below).

Additionally, or cleaner separation of listing available fields vs. fetching field values.

Furthermore, we should set a good example in the code of these sources since people will refer to these files when implementing their own sources.

We also didn't have any test coverage for that source yet.

For reference, #72799 added unit tests for the core/post-meta source, and #73055 for the core/post-data source.

Testing Instructions

Reproduce the following bug in trunk:

• Make sure you don't have any post categories other than the default ("Uncategorized") defined.
• Create a new post, and open the browser console.
• Insert a "Terms Query" block. Select the 'Name' variation to start with.
• Click on the Term Name block that reads "Uncategorized". This will trigger the following warning in the browser console:

The `useSelect` hook returns different values when called with the same state and parameters.
This can lead to unnecessary re-renders and performance issues if not fixed.

Non-equal value keys: core/term-data

Now switch to this PR's branch, and verify that the issue is fixed.

Additionally, perform some smoke testing. The core/term-data source is mostly used for Navigation links to taxonomy term pages, so:

• Create some categories and tags, and add some posts to each of them.
• Make sure you're using a block theme. In the Site Editor, add a new Navigation block to your theme's header template part (or edit an existing Navigation block).
• Add "Category link" and "Tag link" blocks to various categories and tags.
• Save, and view your site on the frontend. Verify that the newly added links work correctly.