Upgrade from diff 4 to diff 5

[cspotcode]

Diff 4 was flagged in some sort of audit at work.

For starters, this audit is wrong: the audit tool got confused and assumed that a devDependency of diff would be used by us. Transitive devDependencies are not installed; this is not how the ecosystem works.

...but all the same, if the upgrade is painless, we might as well do it.

[cspotcode]

Breaking changes in v5 https://github.com/kpdecker/jsdiff/blob/master/release-notes.md#v500