feat(home-manager): rework neovim configuration

Use new neovim option introduced in 0.11

modules/home-manager/cli/neovim/files/lsp/nills.lua

@@ -0,0 +1,15 @@
+---@brief
+---
+-- https://github.com/oxalica/nil
+--
+-- A new language server for Nix Expression Language.
+--
+-- If you are using Nix with Flakes support, run `nix profile install github:oxalica/nil` to install.
+-- Check the repository README for more information.
+--
+-- _See an example config at https://github.com/oxalica/nil/blob/main/dev/nvim-lsp.nix._
+return {
+  cmd = { 'nil' },
+  filetypes = { 'nix' },
+  root_markers = { 'flake.nix', '.git' },
+}