feat(nvim): add base config for nil and nixpkgs-fmt

Still has problem: Your LSP client doesn't support confimration. You can
enable autoArchive in lsp configuration.

flake.nix

@@ -7,6 +7,7 @@
       url = "github:nix-community/home-manager/release-23.11";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    nil.url = "github:oxalica/nil";
     nixpkgs.url = "github:nixos/nixpkgs/nixos-23.11";
     nixpkgs-unstable.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     nixpkgs-vim-enmasse-branch.url = "github:workflow/nixpkgs/vimplugins-enmasse";
@@ -20,7 +21,7 @@
     };
   };
 
-  outputs = { self, nixpkgs, nixpkgs-unstable, nixpkgs-vim-enmasse-branch, nixos-hardware, home-manager, secrets, ... }@inputs:
+  outputs = { self, nil, nixpkgs, nixpkgs-unstable, nixpkgs-vim-enmasse-branch, nixos-hardware, home-manager, secrets, ... }@inputs:
     let
       overlays = {
         nixpkgs-vim-enmasse-branch = import nixpkgs-vim-enmasse-branch {

home/neovim/default.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ pkgs, inputs, ... }:
 let
   vim-enmasse-branch = pkgs.nixpkgs-vim-enmasse-branch;
 
@@ -406,6 +406,8 @@ in
     extraPackages = [
       pkgs.shellcheck
       pkgs.shfmt
+      pkgs.nixpkgs-fmt
+      inputs.nil.packages.x86_64-linux.default # Nix Language Server by Oxalica
     ];
 
     plugins = with pkgs.vimPlugins; [
@@ -648,7 +650,7 @@ in
       vim-numbertoggle
       {
         plugin = mason-nvim; # Automatically install LSP servers
-        config = builtins.readFile ./lsp.lua;
+        config = builtins.replaceStrings ["LSP_PATH"] ["${inputs.nil.packages.x86_64-linux.default}/bin/nil"] (builtins.readFile ./lsp.lua);
         type = "lua";
       }
       {

home/neovim/lsp.lua

@@ -124,7 +124,6 @@ local servers = {
       -- diagnostics = { disable = { 'missing-fields', 'undefined-global' } },
     },
   },
-  -- nil_ls = {}, -- nix
   julials = {},
   pyright = {},
   rust_analyzer = {
@@ -156,3 +155,17 @@ mason_lspconfig.setup_handlers {
     }
   end,
 }
+
+local lsp_path = 'LSP_PATH'
+require('lspconfig').nil_ls.setup {
+  autostart = true,
+  capabilities = capabilities,
+  cmd = { lsp_path },
+  settings = {
+    ['nil'] = {
+      formatting = {
+        command = { "nixpkgs-fmt" },
+      },
+    },
+  },
+}