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This should eliminate page freezes when loading big files/diff.
`highlightBlock` is needed to preserve existing nodes when highlighting
and for that, highlight.js needs access to the DOM API so I added a DOM
implementation to make it work, which adds around 300kB to the output
file size of the lazy-loaded `highlight.js`.
Co-authored-by: Lauris BH <lauris@nix.lv>