Back Compat: Add Patterns submenu for WordPress 6.4

[t-hamano]

Part of #52150
Related core changeset: https://core.trac.wordpress.org/changeset/57543

What?

This PR applies the following changes implemented in WP 6.5 (Changeset 57543) to WP 6.4 and below + Gutenberg plugin environments:

• Add a Patterns submenu (wp-admin/edit.php?post_type=wp_block) under the Appearance menu for the Classic theme
• Allow access to the Site Editor's Patterns page (wp-admin/site-editor.php?path=%2Fpatterns) internally for the Classic theme

Why?

A roadmap for publishing the new Patterns page to the classic theme is mentioned in this comment. We originally thought that we would not be able to publish the new Patterns page to the Classic theme until the minimum version supported by the Gutenberg plugin was WP 6.5. However, we found that by filtering wp_die() itself, it is possible to publish the new Pattern page internally even in environments with WP 6.4 or lower.

This should allow us to officially publish the new Patterns page for the Classic theme in the next major WordPress release (6.6) without worrying about backwards compatibility issues.

How?

I added two hooks to accomplish the same thing with this changeset.

Testing Instructions

• Access the test environment (http://localhost:8889/wp-admin/).
• Enable Twenty Twenty-One or Emptyhybrid themes. Emptyhybrid theme supports block template parts.
• Make sure that there is a Patterns submenu under the Appearance menu, and that it links to the old Patterns page (wp-admin/edit.php?post_type=wp_block).
• Make sure you can access the new Patterns page (http://localhost:8889/wp-admin/site-editor.php?path=%2Fpatterns) directly.
• Reload the browser after navigating pages within the Site Editor. Access should never be denied.
• Make sure that the following URLs cannot be accessed.
  • Design: http://localhost:8889/wp-admin/site-editor.php
  • Default edit view: http://localhost:8889/wp-admin/site-editor.php?canvas=edit
  • Navigation: http://localhost:8889/wp-admin/site-editor.php?path=%2Fnavigation
  • Styles: http://localhost:8889/wp-admin/site-editor.php?path=%2Fwp_global_styles
  • Pages: http://localhost:8889/wp-admin/site-editor.php?path=%2Fpage
  • Templates: http://localhost:8889/wp-admin/site-editor.php?path=%2Fwp_template

Next Step

This PR is purely to enable changes made in WP 6.5 to work in environments below WP 6.4. In order to expose the new Patterns page to the classic theme in WP 6.6, I would like to continue working on the following tasks:

• Gutenberg plugin: Override the Appearance > Patterns submenu to link to the new Patterns page.
• Gutenberg plugin: Point all "Manage all patterns" links to the new Patterns page
• core: Link the Appearance > Patterns submenu to the new Patterns page.

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: youknowriad <youknowriad@git.wordpress.org>

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[aaronrobertshaw]

I'm glad there's a way to make this happen sooner for classic themes. Nice work @t-hamano 🎉

It tests well for me:

✅ Block Themes are not affected
✅ Patterns menu appears under Appearance for classic and hybrid themes
✅ Patterns menu links to wp-admin/edit.php?post_type=wp_block
✅ Site Editor patterns page could be accessed directly
✅ Not denied access to site editor's pattern page when reloading, navigating back etc.
✅ Non-pattern pages in the site editor were not accessible

The code changes reflect the relevant code within core's site-editor.php as well as other existing approaches to adding menu items e.g. Widgets.

LGTM 🚢

[youknowriad]

Nice one indeed :)

[t-hamano]

Thanks for the review!