Add a job to run the ziglings unit tests, using `zig build test`.
Use the new version of actions/checkout, since all Github Actions will begin running on Node16 instead of Node12. See https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.
Add a new github workflow named CI. Add a job named compat, checking that and old Zig compiler will not fail with a compiler error, but instead will print an useful error message.