Move lsp config to local file structure

.config/nvim/lsp/dafny.lua

@@ -0,0 +1,15 @@
+---@brief
+---
+--- Support for the Dafny language server.
+
+-- The default `cmd` uses "dafny server", which works on Dafny 4.0.0+. For
+-- older versions of Dafny, you can compile the language server from source at
+-- [dafny-lang/language-server-csharp](https://github.com/dafny-lang/language-server-csharp)
+-- and set `cmd = {"dotnet", "<Path to your language server>"}`.
+
+---@type vim.lsp.Config
+return {
+  cmd = { 'dafny', 'server' },
+  filetypes = { 'dfy', 'dafny' },
+  root_markers = { '.git' },
+}