programs/nano: Generate nanorc if syntaxHighlight enabled

[avdv]

• prepend a newline to the include directive
• generate the nanorc by default, since cfg.syntaxHighlight is true

Motivation for this change

As a beginner with NixOS I was struggling how to enable syntax highlighting for nano, given that by default it should be already enabled.

To actually get it enabled one has to change the value of config.programs.nano.nanorc to a value different from the empty string.

I did, but it did not work at first, since one has to remember to add a newline to the value since it simply gets concatenated to the include "${pkgs.nano}/share/nano/*.nanorc" config statement.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[Ma27]

Tested in a VM locally, this fixes the syntax highlighting option for nano 👍

[Ma27]

@avdv thanks!