update on package:tuple status

As an update on package:tuple - we consider this package to be feature complete. With Dart 3.0, users now have the ability to use [Records](https://dart.dev/language/records):

> Records are an anonymous, immutable, aggregate type. Like other collection types, they let you bundle multiple objects into a single object.

```dart
var record = (123, true);
print('${record.$1}: ${record.$2}');
```

By and large, Records serve the same use cases that `package:tuple` had been used for. New users coming to this package should likely look at using Dart Records instead. Existing uses of package:tuple will continue to work, however we don't intend to enhance the functionality of this package; we will continue to maintain this package from the POV of bug fixes.