cc @SomeoneToIgnore

## Summary

Follow-up to https://github.com/zed-industries/zed/discussions/55352,
where the conclusion was to split `editor.rs` incrementally by topic
instead of all at once.

This mechanically extracts diagnostics-related editor code into
`crates/editor/src/editor/diagnostics.rs` while preserving the existing
public API via re-exports.

## Testing

- `cargo check -p editor --lib`
- `cargo check -p diagnostics --lib`
- `cargo check -p diagnostics --tests`

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A