Support Drupal 12 Default Admin (Gin-in-core) styling

Problem/Motivation

Drupal 12 ships a Gin based admin theme in core, called Default Admin. The widget only looked for the Gin contrib theme, so on the core admin theme it fell back to the plain styling and lost the tag look. The Gin stylesheet also depended on the --gin-* custom properties being there, with no fallback for the day the core theme renames or drops them.

Steps to reproduce

1. Install Drupal 12 and keep the Default Admin theme.
2. Open a form with an Autocomplete Deluxe field.
3. The Gin styling is not attached and the tags fall back to the plain look.

Proposed resolution

Detect the Gin contrib theme and the core Default Admin theme, and their subthemes, in one place. Give every custom property in the Gin stylesheet a fallback value, move the tag rules to logical properties so right to left follows on its own, and draw the throbber with an animation rather than a background image. The theme detection moves into a small static helper so it can be unit tested without the theme system, and the module also declares Drupal 12 support.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ✅ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ✅ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Developer Documentation
• ➖ User Guide Documentation
• ➖ Reviewed by human
• ➖ Code review by maintainers
• ➖ Full testing and approval
• ➖ Credit contributors
• ➖ Review with the product owner
• ➖ Release notes snippet
• ❌ Release

User interface changes

• The tag styling follows the Gin contrib theme and the Drupal 12 Default Admin theme alike, in both writing directions.

API changes

• The help hook moves to an object oriented hook class, with the procedural function kept as a legacy wrapper for Drupal 10 and 11.0.
• The field widget plugin moves from an annotation to the PHP attribute.

Data model changes

• N/A

Release notes snippet

• Autocomplete Deluxe styles its tags under the Drupal 12 Default Admin theme as it does under Gin, and declares Drupal 12 support.