Add topics to pubspec.yaml

[szakarias]

Topics help 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:

 - my-topic
 - some-other-topic

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

• Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[szakarias]

Moved to g3.