Document highlight refactor

[pfitzseb]

Factor out common code for #908.

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• End-user documentation check. If this PR requires end-user documentation in the Julia VS Code extension docs, please add that at https://github.com/julia-vscode/docs.
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[davidanthoff]

@pfitzseb what is the status of this? It needs a milestone and label etc.

[pfitzseb]

This is basically just a slight refactor for #908 and should be merged at the same time as that PR.

[davidanthoff]

Needs merge conflict resolution.

[davidanthoff]

At the moment tests seem to fail. Not clear to me whether that is actually related to this PR, but tests on master seem to pass, so my best guess is that this is somehow related to this PR?