Set patterns

Currently, there are Map patterns and List patterns, but no Set patterns. I haven't found an issue tracking this, and I believe this would be a nice feature.

For example, in flutter there is `MaterialStateProperty.resolveWith`, which gives you a set of states that you have to resolve to a certain value (such as a color).
Example usage:
```dart
MaterialStateProperty.resolveWith((states) {
return switch(states) {
{MaterialState.selected, MaterialState.disabled} => Colors.grey,
{MaterialState.selected} => Colors.yellow,
_ => Colors.transparent,
};
}
```