languages/nix: Add flake formatter

Add a formatter to the nix language module that runs the `nix fmt`
command, which uses a flakes default formatter.

docs/release-notes/rl-0.8.md

@@ -486,3 +486,5 @@
 - Add [roslyn-ls] to the `vim.languages.csharp` module.
 
 - Added json support under `vim.languages.json` using [jsonls] and [jsonfmt].
+
+- Add flake formatter type to the nix language module.

modules/plugins/languages/nix.nix

@@ -23,6 +23,9 @@
       (mkIf (cfg.format.type == "nixfmt") {
         command = ["${cfg.format.package}/bin/nixfmt"];
       })
+      (mkIf (cfg.format.type == "flake") {
+        command = ["${cfg.format.package}/bin/nix" "fmt"];
+      })
     ];
   };
 
@@ -58,6 +61,10 @@
     nixfmt = {
       package = pkgs.nixfmt-rfc-style;
     };
+
+    flake = {
+      package = pkgs.nix;
+    };
   };
 
   defaultDiagnosticsProvider = ["statix" "deadnix"];