feat: Migrate autocmd in on_attach to pure Lua call

Migrated, following TeeJ_Dv's advice, the auto-command creation to a Lua
function.  As this auto-command only cares for the on_attach Lua scope,
it seems correct to have this one defined as a Lua function instead of
the legacy Vimscript.

nvim/lsp_client_settings.vim

@@ -46,7 +46,13 @@ lua << EOF
 local nvim_lsp = require("lspconfig")
 
 local on_attach = function(client, bufnr)
-  vim.api.nvim_command("autocmd CursorHoldI <buffer> lua vim.diagnostic.open_float({focusable = false, header = ''})")
+  vim.api.nvim_create_autocmd("CursorHoldI", {
+    buffer = bufnr,
+    callback = function()
+      vim.diagnostic.open_float({ focusable = false, header = "" })
+    end
+  })
+
   vim.lsp.handlers["textDocument/publishDiagnostics"] = vim.lsp.with(
     vim.lsp.diagnostic.on_publish_diagnostics, {
       update_in_insert = true,