feat(lang): add nix language support

lua/lazyvim/plugins/extras/lang/nix.lua

@@ -0,0 +1,35 @@
+return {
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = function(_, opts)
+      opts.ensure_installed = opts.ensure_installed or {}
+      vim.list_extend(opts.ensure_installed, { "nix" })
+    end,
+  },
+  {
+    "williamboman/mason.nvim",
+    opts = function(_, opts)
+      vim.list_extend(opts.ensure_installed, {
+        "nixpkgs-fmt",
+        "nil",
+      })
+    end,
+  },
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        ["nil"] = {},
+      },
+    },
+  },
+  {
+    "stevearc/conform.nvim",
+    optional = true,
+    opts = {
+      formatters_by_ft = {
+        nix = { "nixfmt" },
+      },
+    },
+  },
+}