Move GitHub rendering to `ruff_db` (#20320)
## Summary
This is the GitHub analog to #20117. This PR prepares to add a GitHub
output format to ty by moving the implementation from `ruff_linter` to
`ruff_db`. Hopefully this one is a bit easier to review
commit-by-commit. Almost all of the refactoring this time is in the
first commit, then the second commit adds the new `OutputFormat` variant
and moves the file into `ruff_db`. The third commit is just a small
touch up to use a private method that accommodates ty files so that we
can run the tests and update/move the snapshots.
I had to push a fourth commit to fix and test diagnostics without a
span/file.
## Test Plan
Existing tests
