[Full Site Editing]: Include theme's templates in template list in post editor

[ntsekouras]

What?

Fixes: #41617
Regression from: #41387

Why?

In the PR that caused the regression, I overrode the templates that were coming from get_page_templates(core) but that made the php theme templates lost. This PR fixes that by excluding only the needed templates.

Testing Instructions

1. Add a php template in theme's templates directory.

<?php
/**
 * Template Name: Full-width Contained
 */

2. Edit any page
3. Observe that the php templates are in the list

Screenshots or screencast

[carolinan]

I tested classic themes by adding theme support for block-templates to Twenty Twenty.
I tested block themes by adding a PHP template to Twenty Twenty-Two.
The PR solves the problem. The added context in the comment is clear.

[skorasaurus]

Thanks for the quick repair on this; the regression had been affecting me as well; and I can also confirm this PR fixes the issues I experienced.

[mburridge]

Added the Needs Dev Note label in case this needs a dev note (either individual or as part of a "misc" dev note) for WP 6.1 release.

[ntsekouras]

Added the Needs Dev Note label in case this needs a dev note (either individual or as part of a "misc" dev note) for WP 6.1 release.

This was a small regression so the fix restored the previous functionality. No need for dev note.