Make style book available to classic themes

[carolinan]

This PR is the PHP backport of WordPress/gutenberg#69043.
It is not fully functional without the JavaScript changes.

In this alternative for the stylebook in classic themes, all classic themes have a menu under Appearance called Design.
Design opens the preview of the homepage, and the sidebar has two menu items: Patterns and Styles.
Styles opens the Stylebook. This stylebook is not interactive: not yet. It is a preview of the color palette, blocks etc.
In other words, there is no opt-in for the stylebook, it is available by default.

The PR enables access to the stylebook by:

• Registering a new theme support called stylebook. To enable checking for the new theme support in the Site Editor, I needed to make it available in the themes REST API endpoint, which is why I am using https://developer.wordpress.org/reference/functions/register_theme_feature/ with show_in_rest set to true.
• Enabling the theme support by default
• Updating the menu item under Appearance from Patterns to Design, if the theme support is enabled.

In addition, the PR updates the path to the patterns page, following the recent change to the site editor URLs.

Trac ticket:
https://core.trac.wordpress.org/ticket/62509

Testing Instructions

Activate any classic theme.
Go to Appearance > Design > Styles.
The Stylebook should display correctly.

Add remove_theme_support( 'stylebook' ); to the theme, for example in the themes functions.php file, in the theme setup.
Check that the Appearance menu now has Patterns instead of Design.

(This is the part that needs the JavaScript changes: When you activate the Patterns link, and go back one step, you will see that the style book is still available even though the theme support has been removed.)

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