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.

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• 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.
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• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

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