editor: Extract Diagnostics code out of editor.rs

[GoldStrikeArch]

cc @SomeoneToIgnore

Summary

Follow-up to #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:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• N/A

[SomeoneToIgnore]

Great, thank you for the example, the idea seems to be well demonstrated and great if we can process each move sequentially, making some notes between steps if necessary.

Here, some overly public members are somewhat odd to expose: let's check out the usages of each entry moved and see if that could be inlined/made private/etc.
In some cases, cargo does not emit warnings for items that are pub but not used anywhere, so it's nice to make some tidy up if we're onto it anyway.

[GoldStrikeArch]

@SomeoneToIgnore

Thanks, agreed. I tightened this up by:

• removing the public re-exports for diagnostic_style and ActiveDiagnosticGroup
• moving diagnostic_style into display_map as a private helper
• moving the pub(super) diagnostics helpers below the public methods

I kept the remaining public methods because they still have cross-crate callers from diagnostics, git_ui, agent_ui, etc.

[SomeoneToIgnore]

Nice, thank you.