This PR sets up an "ecosystem" check as an optional part of the CI step for pull requests. The primary piece of this is a new script in `scripts/check_ecosystem.py` which takes two ruff binaries as input and compares their outputs against a corpus of open-source code in parallel. I used ruff's `text` reporting format and stdlib's `difflib` (rather than JSON output and jsondiffs) to avoid adding another dependency. There is a new ecosystem-comment workflow to add a comment to the PR (see [this link](https://securitylab.github.com/research/github-actions-preventing-pwn-requests/) which explains why it needs to be done as a new workflow for security reasons).
