Warning when a registerFallbackValue is never used

**Description**
As a user I would like to get a warning whenever a fallback value has been registered but has never been used.

```dart
void main() {
setUpAll(() {
registerFallbackValue(Foo()); // Should result in a warning since it is never used.
});
}
```

**Possible solutions**
- Runtime warning
- Custom analyser lint rule

**Describe alternatives you've considered**
N/A

**Additional context**
- https://invertase.io/blog/announcing-dart-custom-lint