feat(lang): add nix language support (#2956)

In the future we can add:
- dap / debugger support, if this is ever possible or needed.
- extend neotest, if this is ever needed, for most people I suspect not
needed.
- linting, if this is ever needed.
- ~~maybe enhance nvim-cmp behavior with nixpkgs, rust lang extension
crates.nvim does smt with nvim-cmp, other language extensions don't.~~

I think its already good to merge this PR, we can add these enhancements
later, if they are ever needed.

---------

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

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

@@ -0,0 +1,30 @@
+return {
+  recommended = {
+    ft = "nix",
+    root = "flake.nix",
+  },
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = function(_, opts)
+      opts.ensure_installed = opts.ensure_installed or {}
+      vim.list_extend(opts.ensure_installed, { "nix" })
+    end,
+  },
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        nil_ls = {},
+      },
+    },
+  },
+  {
+    "stevearc/conform.nvim",
+    optional = true,
+    opts = {
+      formatters_by_ft = {
+        nix = { "nixfmt" },
+      },
+    },
+  },
+}