Refactor diff_view to include diff_line

What does this MR do and why?

This MR solves: #364826 (closed)

It refactors the diff_view component and introduces a diff_line to clean things up and use the benefits of computed values.

Screenshots or screen recordings

screenshot (no visual changes)

How to set up and validate locally

1. make sure your GDK has an ultimate license
2. Feature.enable(:refactor_code_quality_inline_findings)
3. clone this project: https://gitlab.com/jannik_lehmann/code-quality-test
4. recreate this MR: http://gdk.test:3000/root/code-quality-test-index/-/merge_requests/1
5. run pipeline
6. See inline diffs in diff view of created MR

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.