Move lsp config to local file structure

.config/nvim/lsp/nil_ls.lua

@@ -0,0 +1,17 @@
+---@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._
+
+---@type vim.lsp.Config
+return {
+  cmd = { 'nil' },
+  filetypes = { 'nix' },
+  root_markers = { 'flake.nix', '.git' },
+}