desktop
Enable "experimental" diff viewer for unified diffs
Description
This PR enables in beta builds the new diff viewer used for side-by-side diffs in unified diffs, replacing CodeMirror. This is something we intended to do for a long time, since we expect our implementation to be easier to make accessible, but we were blocked by our inability to select text properly in long files. Something that was solved by @niik in #15128
In this PR, aside from enabling it, I made a few CSS changes to ensure it looks as good as the current version. I also found a bug introduced in #15128 regarding which diff to use when copying the text (props.diff vs state.diff… not sure how it worked to begin with 🤔 because the diff from props is the original diff, while the one from state is the expanded diff -or the original diff if it was never expanded by the user).
Release notes
Notes: no-notes
