Adds two configuration-file only settings `extend-safe-fixes` and
`extend-unsafe-fixes` which can be used to promote and demote the
applicability of fixes for rules.
Fixes with `Never` applicability cannot be promoted.
## Summary
This PR moves `Diagnostic`, `DiagnosticKind`, and `Fix` into their own crate, which will enable us to further split up Ruff, since sub-linter crates (which need to implement functions that return `Diagnostic`) can now depend on `ruff_diagnostics` rather than Ruff.
