Add horizontal scroll for diffs with long lines

[Karrq]

Hello!

I encountered the same annoyance as #1048 so I decided to get a clanker to make a fix for it.

To be honest, I only reviewed it in the "all files" view and the single file one, but not in the review story or other views.
I will try it locally a bit more and unmark as draft once I see it works in more views :)

[Karrq]

Works well, but word wrap mode is sufficient for my needs. I recommend others to try it

[backnotprop]

Merged as #1586, thanks @Karrq. Your code shipped unchanged apart from a rebase onto main, dropping the lockfile drift, and re-pinning the viewer manifest; the PR was opened from this repo only because maintainer edits were off on your branch. Closes #1566 and #1048. Ships in the next release.