Add topics to pubspec.yaml

[jonasfj]

Topics helps users on pub.dev discover packages, and related packages.

You can see a list of topics here.
You can also make your own topics.

Topics are added to pubspec.yaml as follows:

topics:
 - my-topic
 - some-other-topic

See also: https://dart.dev/tools/pub/pubspec#topics

[Bungeefan]

Thanks for the PR and the information about pub.dev topics!