nixos/monit: change type of 'config' option to lines

[ryantm]

By using types.lines for 'config', we can specify monit configurations
in lots of modules and they can all be automatically combined together
with newlines. This is desireable because different modules might want
to each specify the small monitoring task specific to their service.

This commit also updates the module to use current idioms.

Motivation for this change

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)
• Fits CONTRIBUTING.md.

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