Add weeder (dead code elimination tool) to dev environment

[fisx]

https://wearezeta.atlassian.net/browse/WPB-9667

i've run weeder from a PR locally with success (>1400 weeds, but many of them are false positives). now we only need to add that patch to our nix envs. (we should probably not run it on ci for now, until we have fine-tuned weeder.toml and eliminated all the legitimate weed.)

the purpose of this PR is:

• add weeder to the dev env so people can run it and fine-tune it

the purpose of this PR is NOT:

• add weeder to the CI (see https://github.com/NorfairKing/weeder-nix, but different PR)
• remove any weed
• have a complete list of roots in the weeder.toml
• push all the buttons that make weeder as strict as possible

Checklist

• [x] Add a new entry in an appropriate subdirectory of changelog.d
• [x] Read and follow the PR guidelines

[fisx]

This doesn't install the pinned version of weeder, but the one from hackage (2.7).