feat(generator): support map-like and pair-iterable types

- Automatically generate `JSIterable` implementations for types defined as iterable in Web IDL.
- Generate `asMap` views for map-like types in `js_interop_gen` (e.g., `URLSearchParams`, `FormData`, `Headers`).
- Generate `toDart` getters for pair-iterable types like `XRHand`.
- Bump minimum SDK constraint to `^3.12.0-0` across packages to support new features like private named parameters.
- Update CI workflows to test on `beta` and `dev` instead of `3.10`.
- Fix test failures related to SDK version string checks.
- Clean up refactorings in `translator.dart` to preserve `followedBy` style for review.