6365: Do insertion lookahead in algo::diff r=matklad a=Veykril
This is the last blocker for #6287 after this I can update that PR to properly fix things through using `SyntaxRewriter`.
This PR also shuffles tests around a bit and adds some more.
Ideally this is just a hack until we implement a "proper" diff algorithm that approximates a minimal diff. Maybe something like [gumtree](https://github.com/GumTreeDiff/gumtree)?
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
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