Add markdownlint

[declension]

Is your feature request related to a problem? Please describe.

The supplied mdformat is good for a lot of use cases, and if you agree with its strong / unconfiguable opinions, but there are other packages with a lot more flexibility out there.

e.g. https://github.com/DavidAnson/markdownlint

Like Shellcheck, it supports and publishes a lot of configurable rules and can auto-fix many of these.

Describe the solution you'd like

settings.markdownlint.enable = true, I guess. Maybe some thought about how to load .markdownlint.yaml (or equivalent)

Describe alternatives you've considered

The existing MD linter as described

Current workaround
Probably could be improved, but adding this to settings.formatter

    markdownlint = {
      command = "${pkgs.bash}/bin/bash";
      options = [
        "-euc"
        ''
          # Needed to pick up config
          cd ${./.}
          ${pkgs.lib.getExe' pkgs.nodePackages.markdownlint-cli2 "markdownlint-cli2"} --fix $@
        ''
        "--" # bash swallows the second argument when using -c
      ];
      includes = ["*.md"];
    };

[zimbatm]

Sounds good, feel free to send a PR.

treefmt executes the formatter with the project root as the current directory, so the wrapper shouldn't be needed. Alternatively, you might want to use the "settings" attribute to generate the config file with nix and pass it with --config-file to the program.