[ALL] Consider adding `topics` to `pubspec.yaml` for better discoverability

[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

[russellwheatley]

Hi @jonasfj - thank you for bringing this to our awareness, I'll report back to the team and see what topics we'd like to add to each package 👍