Asciidoctor: add pygments.rb (highlighter) to Gemfile

[PierreR]

Motivation for this change

I use pygments.rb to highlight haskell code in adoc file. Maintaining local gem files just to get that feature is a bit of a pain.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@PierreR, thanks for your PR! By analyzing the history of the files in this pull request, we identified @gpyh and @k0ral to be potential reviewers.

[PierreR]

@zimbatm would you mind reviewing this PR ?

[zimbatm]

Sure. Can you add me a sample where pygments.rb is being used? If I remember correctly there are multiple highlighting options available, is pygments.rb the best one?

[PierreR]

@zimbatm indeed there are a couple of source highlighter for asciidoctor:

• highlightjs: builtin (don't require any extra gem) but won't render in pdf
• coderay: requires a separate gem, less languages supported
• pygments : requires a separate gem, many languages supported
• prettify: don't know much about it but it didn't match my requirements at the time. I am not sure it is still active.

My personal requirement is to have haskell supported in html and pdf. AFAIK only pygments can do that.

As a extra note, the PR just add pygments as a possible choice. You can still use hightlightjs if you wish so.

We can either include all possible highlighters or wait for a request before doing so.

[zimbatm]

Thanks. Let's go with the batteries included route.