editorconfig: Add editorconfig plugin (#265)

This is temporary, until neovim 0.9 when we will have builtin editorconfig

plugins/utils/editorconfig.nix

@@ -0,0 +1,59 @@
+{
+  lib,
+  config,
+  pkgs,
+  ...
+} @ args:
+with lib; let
+  # TODO: Migrate to builtin editorconfig in neovim 0.9
+  helpers = import ../helpers.nix args;
+in {
+  options.plugins.editorconfig = {
+    enable = mkEnableOption "editorconfig plugin for neovim";
+
+    package = helpers.mkPackageOption "editorconfig.nvim" pkgs.vimPlugins.editorconfig-nvim;
+
+    properties = mkOption {
+      type = types.attrsOf types.str;
+      default = {};
+      description = ''
+        The table key is a property name and the value is a callback function which accepts the
+        number of the buffer to be modified, the value of the property in the .editorconfig file,
+        and (optionally) a table containing all of the other properties and their values (useful
+        for properties which depend on other properties). The value is always a string and must be
+        coerced if necessary.
+      '';
+      example = {
+        foo = ''
+          function(bufnr, val, opts)
+            if opts.charset and opts.charset ~= "utf-8" then
+              error("foo can only be set when charset is utf-8", 0)
+            end
+            vim.b[bufnr].foo = val
+          end
+        '';
+      };
+    };
+  };
+
+  config = let
+    cfg = config.plugins.editorconfig;
+  in
+    mkIf cfg.enable {
+      extraPlugins = [cfg.package];
+
+      extraConfigLua = let
+        mkProperty = name: function: ''
+          __editorconfig.properties.${name} = ${function}
+        '';
+        propertiesString = lib.concatLines (lib.mapAttrsToList mkProperty cfg.properties);
+      in
+        mkIf (propertiesString != "" && cfg.enable) ''
+          do
+            local __editorconfig = require('editorconfig')
+
+            ${propertiesString}
+          end
+        '';
+    };
+}