refactor(lua): improve type annotations

runtime/lua/vim/lsp/handlers.lua

@@ -646,6 +646,7 @@ M[ms.window_showMessage] = function(_, result, ctx, _)
   if message_type == protocol.MessageType.Error then
     err_message('LSP[', client_name, '] ', message)
   else
+    --- @type string
     local message_type_name = protocol.MessageType[message_type]
     api.nvim_out_write(string.format('LSP[%s][%s] %s\n', client_name, message_type_name, message))
   end