Consider adding `topics` to `pubspec.yaml` for better discoverability

Topics helps users on pub.dev discover packages, and related packages.

You can see a list of [topics here](https://pub.dev/topics).
You can also make your own topics.

Topics are added to `pubspec.yaml` as follows:

```yaml
topics:
 - my-topic
 - some-other-topic
```

See also: https://dart.dev/tools/pub/pubspec#topics