Update the opt-in for the stylebook in classic themes

[carolinan]

What?

This PR changes how classic themes can opt-in to show the stylebook under the menu Appearance > Design.

Previously the stylebook was shown when the theme included add_theme_support( 'editor-styles' );
The PR replaces the check for editor-styles with current_theme_supports( 'editor', 'stylebook' )

What this does not do:
The PR does not offer a way to disable the style book for classic themes that add theme.json.
To do: Figure out what to do about the direct path to the stylebook
wp-admin/site-editor.php?p=%2Fstylebook

Partial for #68036

Why?

• Theme developers may want to use editor-styles without the stylebook. A use case would be when the developer has created its own style guide.
• Theme developers may be unaware that using the existing theme support enables new features.
• By adding a new 'editor' theme support, theme developers can explicitly opt-in to the stylebook.
  The new editor theme support can be extended later, as an alias for existing theme support related to the Site Editor, and for new features.

How?

The PR replaces the check for editor-styles with current_theme_supports( 'editor', 'stylebook' )

Testing Instructions

There should be no change for block themes and hybrid themes:
classic themes with theme.json
classic themes with templatest/index.html

Activating a classic theme should not enable the Design menu unless the theme support is added.

To add the theme support, copy paste the following into the theme setup function, usually in functions.php:

		add_theme_support(
			'editor',
			array(
				'stylebook'
			)
		);

After it is added, the design menu item with the nested Patterns and Styles pages should be available.
The Pattern menu item should not be showing directly under Appearance.

[github-actions]

Flaky tests detected in dc249b2.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/13029645509
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[carolinan]

I added the needs backport label, but I see that the PR for the trac ticket from the previous PR is still a draft: WordPress/wordpress-develop#7865

And is conflicting with the new site editor paths: https://github.com/WordPress/wordpress-develop/pull/7903/files

[Mamaduka]

@carolinan, what are the blockers for this PR?

[carolinan]

#68959
and that the core patch has not been started on.

[github-actions]

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[Mamaduka]

#68959 doesn't have to block this. It's a pre-existing bug here we're just changing the theme supports flag.

[carolinan]

Changing the theme support here only changes the menu item, nothing else, and if that is what we want to do, no, a theme support is not needed at all because developers can already change the menu items.

[Mamaduka]

Then I misunderstood the #68036. I thought the request was to use different theme support flags and make this feature opt-in for classic theme developers, which this PR does.

The new site redirection logic is a separate enhancement; related issues will be resolved separately.

a theme support is not needed at all because developers can already change the menu items.

Developers can do a lot, but frameworks still provide APIs to make certain things more manageable.

[carolinan]

I don't understand. It does not opt-in to the style book to me. It opts in to the menu item.
But if I am the only blocker, go ahead.

[carolinan]

What should have been obvious, is that there is also a Styles menu item in the Site Editor Sidebar, that can be found if you open the Patterns page and then go back to the start of the Site Editor. This menu item is not removed by changing the Admin menu 😆

If the decision is to opt-in to the stylebook, then the theme support needs to be registered so that it is available in the Site Editor. Then the entire Stylebook can be shown conditionally.

[carolinan]

I am closing this in favour of #69043
69403 registers the theme support, which is closer to what we need to do in order to show or hide the Site Editor menu item and stylebook page.