chore: add topics to `pubspec.yaml` #2453

Job	Run time
examples (packages/flutter_bloc/example)	58s
examples (examples/flutter_bloc_with_stream)	1m 47s
examples (examples/flutter_complex_list)	1m 23s
examples (examples/flutter_counter)	1m 18s
examples (examples/flutter_dynamic_form)	1m 28s
examples (examples/flutter_firebase_login)	2m 7s
examples (examples/flutter_form_validation)	1m 4s
examples (examples/flutter_infinite_list)	1m 20s
examples (examples/flutter_login)	1m 26s
examples (examples/flutter_shopping_cart)	1m 31s
examples (examples/flutter_timer)	1m 25s
examples (examples/flutter_todos)	2m 1s
examples (examples/flutter_todos/packages/local_storage_todos_api)	1m 8s
examples (examples/flutter_todos/packages/todos_api)	1m 1s
examples (examples/flutter_todos/packages/todos_repository)	1m 6s
examples (examples/flutter_weather)	1m 34s
examples (examples/flutter_wizard)	1m 3s
examples (examples/github_search/flutter_github_search)	59s
build	1s
	24m 40s

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