Make weight classes implements an interface
I would do something like 
```dart
PhosphorIcon((state.saved
                                ? PhosphorIcons.fill
                                : PhosphorIcons.light)
                            .floppyDisk)
```
but PhosphorIcons.fill and PhosphorIcons.light doesn't share the same interface.
Maybe an abstract class `PhosphorFontSet` that has these abstract properties