Init elm-instrument

• This package is not that useful on its own but it's a first logical step for packaging elm-coverage
• PR to upstream we're currently using as a patch

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @domenkozar this is part 1/2 of bringing elm-coverage in. I think doing it one by one will result in clearer diff in PRs. Alternatively I can continue on node.js side within this PR.