Setup Swiftlint on GitHub Actions to run on PRs

Attempted at #113 using:

- https://github.com/marketplace/actions/github-action-for-swiftlint
- https://github.com/norio-nomura/action-swiftlint

Unfortunately, that action only runs on linux. So it won't work for us.

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We could implement a very basic version by just running `./scripts/lint.zsh`.

SwiftLint is pre-installed on macOS runners.