This merges `shared_preferences_ios` and `shared_preferences_macos` into a new `shared_preferences_foundations` that replaces both of those packages, as described in https://github.com/flutter/flutter/issues/117941:
- Renames `shared_preferences_macos` to `shared_prefrences_foundation`, since the macOS implementation is the Swift implementation, which is what we want to use going forward.
- Moves the implementation files to a shared directory (called `darwin/` in anticipation of https://github.com/flutter/flutter/pull/115337), adjusting the code and podspec slightly to make it iOS-compatible
- Adds iOS support to the example, via `flutter create`-ing a new iOS example and wiring it up to use the existing native unit test. (This was done instead of moving the example from `shared_preferences_ios` since it seemed better to have the example be in Swift as well now.)
- Removes `shared_preferences_ios`.

Once this lands and has been published, a follow-up will update `shared_preferences` to use this new package instead of the other two, and the old ones will be marked as deprecated on pub.dev.

Part of https://github.com/flutter/flutter/issues/117941