Support for line highlighting in code blocks

[stefano-ottolenghi]

(Color comes from Needle, but cannot use the css color vars directly because they embed an opacity of 1, which we want to be 0.5)

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[recrwplay]

We'll need to make sure that with the highlight colors used for text in codeblocks, the background color choice passes WCAG AA (and ideally AAA).

This PR doesn't materially affect A11y status though, since we need to update the text highlight colors anyway.

If we want to merge this we should note that the colors will need to change later.

[stefano-ottolenghi]

Thanks for raising this. If colors will need to change anyway, let's merge this and keep it in mind for the future.

[stefano-ottolenghi]

There seems to be a glitch where sometimes the // mark-line is not parsed and is displayed as-is. I suspect a bug in highlight.js, possibly this one highlightjs/highlight.js@970a4d3 , which is unfortunately only fixed in v11.