fish: add pluginModule type

Similar to zsh's `pluginModule` type, but without an initialization
file.

modules/programs/fish.nix

@@ -6,6 +6,29 @@ let
 
   cfg = config.programs.fish;
 
+  pluginModule = types.submodule ({ config, ... }: {
+    options = {
+      src = mkOption {
+        type = types.path;
+        description = ''
+          Path to the plugin folder.
+          </para><para>
+          Relevant pieces will be added to the fish function path and
+          the completion path. The <filename>init.fish</filename> and
+          <filename>key_binding.fish</filename> files are sourced if
+          they exist.
+        '';
+      };
+
+      name = mkOption {
+        type = types.str;
+        description = ''
+          The name of the plugin.
+        '';
+      };
+    };
+  });
+
   abbrsStr = concatStringsSep "\n" (
     mapAttrsToList (k: v: "abbr --add --global ${k} '${v}'") cfg.shellAbbrs
   );