chore(audoedit): simplify diff utils and renderer data structures

[valerybugakov]

• No functional changes.
• Simplifies data structures used to calculate the diff between the current document content and predicted edits.
• Simplifies data structures used to render autoedit decorations.
• Removes redundant data structures and transformations happening between the diff calculation and the renderer call.
• Adds granular changes: LineChange to ModifiedLineInfo, representing individual insertions and deletions in modified lines. This is helpful for troubleshooting purposes, and I plan to use it for the experimental inline renderer implementation that we discussed recently in Slack.
• Updates the default autoedits renderer to use all the above changes.
• Built on top of https://github.com/sourcegraph/cody/pull/6163

Test plan

Manually tested; updated existing diff-utils unit tests and added new ones to cover LineChange calculations.

[valerybugakov]

@hitesh-1997, I gave it my best shot with manual testing (which is how I found https://github.com/sourcegraph/cody/pull/6171), but this is still a pretty sensitive area without any tests yet (referring to the default renderer). Let me know if you spot anything during your manual testing!

[hitesh-1997]

@hitesh-1997, I gave it my best shot with manual testing (which is how I found #6171), but this is still a pretty sensitive area without any tests yet (referring to the default renderer). Let me know if you spot anything during your manual testing!

This makes sense @valerybugakov Played with the PR for sometime, I didn't find anything breaking with manual testing but since we are starting to make more frequent changes, we should definitely prioritize tests, will try to wrap up the diff logic PR from my end so that we can move to testing :)