Block Themes: Update the theme folders to be templates and parts

[youknowriad]

This ticket is here to track the backport of ​https://github.com/WordPress/gutenberg/issues/36548 implemented in ​https://github.com/WordPress/gutenberg/pull/36647

The idea is to use "templates" and "parts" as folders instead of "block-templates" and "block-template-parts" for block themes while retaining backward compatibility for the latter.

While there are already themes that use the previous folders here, we prefer to update the folders while the number of themes is small 20/30 early adopters to ensure consistency in theme folders for all FSE themes (we expect a far bigger number in the future).

Existing FSE themes will continue to work properly without any change, backward compatibility for the previous folders will be kept.

Delaying this change to 6.0 would be more impactful since more themes would exist at that point and plugins might start relying on the presence of the old folders.

Backport the theme folders change from the Gutenberg plugin.

Trac ticket: https://core.trac.wordpress.org/ticket/54493

Gutenberg counterpart PR: ​https://github.com/WordPress/gutenberg/pull/36647

This LGTM. Unfortunately, I can't currently test manually (some issue with MySQL in my local dev setup, probably yet another collision with my GB dev setup 🙄 ).

Instead, I've tested these changes on top of #1920 (by running unit tests), and they're unfortunately failing 😕

There were 3 failures:

1) Block_Template_Test::test_child_theme_block_template_takes_precedence_over_equally_specific_parent_theme_php_template
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/var/www/src/wp-includes/template-canvas.php'
+'/var/www/tests/phpunit/includes/../data/themedir1/block-theme/page-1.php'

/var/www/tests/phpunit/tests/block-template.php:153

2) Block_Template_Test::test_custom_page_block_template_takes_precedence_over_all_other_templates
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'Content'
+'
+<p>Page Template</p>
+'

/var/www/tests/phpunit/tests/block-template.php:193

Would it make sense to get #1920 in first for added confidence?

[youknowriad]

In 52247:

Themes: Update the base folders for templates and template parts in block themes.

Block Themes should now use the following folders:

• templates instead of block-templates
• parts instead of block-template-parts

Existing themes and folders will continue to work without impact.

Props bernhard-reiter.
Fixes #54493.

committed in https://core.trac.wordpress.org/changeset/52247

[youknowriad]

Templates parts using the "parts" folder are still not working on the frontend but this will be fixed automatically in the next package update.