Require grouped Ionic list items

[rdlabo]

Summary

• add the recommended require-ion-item-group template rule for iOS 26 and Material Design 3 list structure
• require ion-list items to use ion-item-group, ion-reorder-group, ion-radio-group, or ion-accordion within ion-accordion-group
• treat Angular control-flow blocks, ng-container, and ng-template as transparent while rejecting intervening rendered elements
• cover @empty and the @defer placeholder, loading, and error blocks
• wrap safely groupable list contents in one ion-item-group with --fix when the same template already uses ion-item-group; otherwise offer a suggestion that reminds users to add the standalone import
• skip fixes for mixed, nested, dynamic, reusable-template, or otherwise ambiguous rendered content
• share Angular template AST traversal across the new rule and existing template rules, including all control-flow secondary blocks and duplicate-node protection
• publish the rule through generated exports/config and document it in README and rule docs

Verification

• npm run update (idempotent)
• npm run lint
• npm test (27 suites, 575 tests)