Add create taxonomies beta in editor sidebar

[faisalahammad]

Adds an opt-in beta feature that lets admins create a taxonomy without leaving the post editor. The panel lives in the document settings sidebar and shows only the essential settings: singular label, plural label, taxonomy key and the hierarchical toggle. The new taxonomy is bound to the post type currently being edited, so it is ready to use on that screen straight away.

This covers the creation half of the request in #155. Editing and deleting taxonomies still happen on the existing Taxonomies screen, which is unchanged.

How it works

A new REST route, POST /scf/v1/taxonomies, handles the create request. It reuses the same input mapping, capability check and validation rules as the classic admin save path in admin-taxonomy.php, then persists through acf_update_internal_post_type(). Reserved keys (post_tag, category, nav_menu, and the other built in ones), duplicate keys and invalid keys are rejected with a specific error, so the sidebar does not need to duplicate that logic to stay safe.

The sidebar UI is a small plugin registered with PluginDocumentSettingPanel, so it sits with the other document settings rather than a modal. The key is derived from the singular label until the user edits it, and the panel is hidden on internal wp_* entities and on post types that are not shown in the admin.

The classic creation flow is untouched, and the whole feature is behind the create_taxonomies beta flag.

Beta registry fix

The beta flag could not be turned on before this PR. include_beta_features() referenced a class file that does not exist, and register_beta_features() registered nothing, so the Features screen had no entries to toggle. That path is fixed here so the new flag actually works. Registering the features also brings back the existing editor_sidebar entry, which had the same problem.

How to test

1. Go to Field Groups > Beta Features and enable "Create Taxonomies in the Editor".
2. Open any post for editing. A "Create Taxonomy" panel appears in the document settings sidebar.
3. Type a singular label such as "Book" and check the key field fills in as book.
4. Fill in the plural label and click Create Taxonomy.
5. The editor reloads and the taxonomy is created and bound to that post type. Confirm it in the Taxonomies screen.
6. Try a reserved key like category or an existing key, and confirm the panel shows a clear error instead of creating anything.
7. Turn the beta feature off and confirm the panel is gone and POST /wp-json/scf/v1/taxonomies returns 404.

Tests run

• composer test:php - full PHPUnit suite, 2953 tests pass. New coverage in tests/php/includes/rest-api/SCFRestCreateTaxonomyTest.php for the endpoint (success, duplicate, reserved, invalid key, unknown post type, permissions, and the route being absent when the beta is off).
• composer test:phpstan - clean on the changed files.
• composer lint:php - clean on the changed lines.
• npm run test:unit - full Jest suite passes, including the new validation tests.
• npm run build - builds without errors.

See #155

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