chore: add topics to `pubspec.yaml` #2452

Job	Run time
examples (packages/flutter_bloc/example)	57s
examples (examples/flutter_bloc_with_stream)	1m 18s
examples (examples/flutter_complex_list)	1m 54s
examples (examples/flutter_counter)	1m 22s
examples (examples/flutter_dynamic_form)	1m 23s
examples (examples/flutter_firebase_login)	1m 42s
examples (examples/flutter_form_validation)	1m 10s
examples (examples/flutter_infinite_list)	1m 39s
examples (examples/flutter_login)	1m 36s
examples (examples/flutter_shopping_cart)	1m 38s
examples (examples/flutter_timer)	1m 22s
examples (examples/flutter_todos)	1m 50s
examples (examples/flutter_todos/packages/local_storage_todos_api)	1m 13s
examples (examples/flutter_todos/packages/todos_api)	56s
examples (examples/flutter_todos/packages/todos_repository)	1m 8s
examples (examples/flutter_weather)	1m 34s
examples (examples/flutter_wizard)	1m 3s
examples (examples/github_search/flutter_github_search)	59s
build	1s
	24m 45s

Provide feedback