Use var/final consistently across repository

[parlough]

Before this change, only dwds was set up to prefer final, while the other projects used an inconsistent mix of var and final. This PR pulls the declaration of prefer_final_locals out to the shared analysis_options.yaml, adds prefer_final_in_for_each as well, and applies the fixes across the repository.

This will help reduce the question of which keyword to use when working in this repository.

[bkonyi]

Nice! Thank you!

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