flake-parts/setups: add a branch option for NixVim instance submodule

modules/flake-parts/setups/home-manager.nix

@@ -151,7 +151,7 @@ let
         (lib.mkIf (config.nixvim.instance != null)
           ({ lib, ... }: {
             imports = [
-              inputs.nixvim.homeManagerModules.nixvim
+              inputs.${config.nixvim.branch}.homeManagerModules.nixvim
             ];
 
             config.programs.nixvim = { ... }: {

modules/flake-parts/setups/nixos.nix

@@ -361,7 +361,7 @@ let
             setupConfig = config;
           in
           { lib, ... }: {
-            imports = [ inputs.nixvim.nixosModules.nixvim ];
+            imports = [ inputs.${config.nixvim.branch}.nixosModules.nixvim ];
 
             programs.nixvim = { ... }: {
               enable = lib.mkDefault true;

modules/flake-parts/setups/shared/nixvim-instance-options.nix

@@ -11,6 +11,22 @@
       '';
     };
 
+    branch = lib.mkOption {
+      type = lib.types.nonEmptyStr;
+      default = "nixvim";
+      example = "nixvim-stable";
+      description = ''
+        The branch of NixVim to be used for the module.
+
+        ::: {.tip}
+        A rule of thumb for properly setting up NixVim with the wider-scoped
+        environment is it should match the nixpkgs version of it. For example,
+        a NixOS system of `nixos-23.11` nixpkgs branch should be paired with a NixVim
+        branch of `nixos-23.11`.
+        :::
+      '';
+    };
+
     additionalModules = lib.mkOption {
       type = with lib.types; listOf raw;
       default = [ ];