Allow interface as its own construct (short for `abstract interface class`)

You should be allowed to declare an `interface`

```dart
interface MyInterface {}
```

Which would be short for:

```dart
abstract interface class MyInterface {}
```

This is similar to how [mixin](https://dart.dev/language/mixins) allows both a separate construct and a class modifier.

---

_Originally this proposal included addition restrictions on interfaces, but those ideas have been moved to a lint proposal instead (https://github.com/dart-lang/sdk/issues/59466)._

Start by comparing the proposed `interface MyInterface {}` syntax with the existing `abstract interface class MyInterface {}` form and the linked Dart mixin construct. Review the proposal's relationship to the moved lint proposal, then determine the language-specification and implementation work needed for the shorthand to behave equivalently.

Written by the indexing model from the issue text.