Fix deprecations and update generated interfaces

[peter-jerry-ye]

Summary

• Updated .gitignore to include target and _build for future migration readiness
• Refactored iterator-related APIs to align with Iterator/Iterator2 changes
• Refreshed generated .mbti interfaces across packages after formatting and updates

Why

• Address deprecated warnings and breaking changes, especially the Iter → Iterator and Iter2 → Iterator2 transition
• Ensure the project remains compatible with current MoonBit tooling and conventions

Notable details

• iter2 functions now delegate to new iterator2 implementations that return Iterator2
• Iterator construction now uses the new callback-based Iterator::new / Iterator2::new contract
• .mbti updates reflect interface changes and are consistent with formatting and public API adjustments