WIP: Add priority countries feature for country fields
- Add field settings for priority countries configuration
- Add priority_countries, show_priority_separator, priority_separator_text settings
- Implement getPrioritizedOptions() method in CountryItem
- Extend CountryDefaultWidget to support priority countries with optgroup separator
- Keep CountryAutocompleteWidget simple (no prioritization needed for autocomplete)
- Add schema definitions for new settings
- Add update hook for existing installations
Known limitations:
- Radio buttons widget (options_buttons) not yet supported
- Separator styling needs CSS library
Addresses #3490668
Co-Authored-By: Claude noreply@anthropic.com