htmlburger/carbon-fields — reverse-engineered prompt

Build me a WordPress custom fields library like Carbon Fields.

I want developers to be able to install it with Composer and quickly add custom fields inside the WordPress admin without writing a full plugin from scratch. It should let them define fields for posts, pages, custom post types, taxonomy terms, users, comments, options pages, navigation menus, and widgets.

Keep it developer friendly, with a clean PHP API, sensible examples, and a quickstart that shows how to register fields and display saved values in a theme. It should work with older WordPress projects too, so don’t assume only the newest PHP version.

Please include the core library code, admin assets if needed, tests, documentation basics, and translation support. Make it feel polished enough that someone could use it as a real open source WordPress package.