`prefer_const_literals_to_create_immutables` support for extension types

Related to dart-lang/sdk#59275, 

```dart
extension type E(List<int> i) { }

var e = E([1]);
```

should we treat `E` as immutable and produce a [`prefer_const_literals_to_create_immutables`](https://dart.dev/tools/linter-rules/prefer_const_literals_to_create_immutables) lint on `[1]`?

/fyi @goderbauer @a14n @eernstg 