Run pre-commit for all files and add instructions for installing pre-commit to dev environments

[raebot]

Currently, the pre-commit checks only run on Github Actions for changes to _shared_utils. In my work migrating away from siuba and other tech debt cleanup, I've noticed a ton of files in this repo with unused imports and unsorted imports. Since the hook hasn't been running for any files outside of _shared_utils these have really stacked up. In addition to checking for and, when possible, auto-correcting unused and unsorted imports, pre-commit will run black for consistent code style, lint with flake8, check that files have an end-of-file newline, and encourage other standard code recommendations that will make for easier collaboration on the team.

I've configured the GH Action workflow to on run pre-commit on files that have changed in the pull request. Without that, it was running against all files in the repo and produced auto-corrections for 800+ files, which would be impossible to review. 😬 This way, we're only running it on files as we're working in those files–a lot more manageable.