New rule: use_descriptive_names_for_type_parameters (when there's 3 or more)

BAD:
```dart
class SomeClass<T, U, K> {
  ...
}
```

GOOD:

```dart
class SomeClass<$firstParam, $secondParam, $thidParam> {
  ...
}
```

GOOD:
```dart
class OtherClass<T> {
  ...
}
```