REST API: Declare 'edit_css' capability in links within WP_REST_Global_Styles_Controller.
Updates the Global Styles endpoint to expose the `'edit_css'` capability via action links.
References:
* [https://github.com/WordPress/gutenberg/pull/46815 Gutenberg PR 46815] Part of an effort to hide custom CSS setting for users without `'edit_css'` capability.
Follow-up to [52342], [52051].
Props mamaduka, dsas, glendaviesnz, mmtr86, talldanwp, timothyblynjacobs.
Fixes #57526.
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git-svn-id: http://core.svn.wordpress.org/trunk@54710 1a063a9b-81f0-0310-95a4-ce76da25c4cd