startup: wait for embedder before executing startup commands and files

Give embeders a chance to set up nvim, by processing a request before
startup. This allows an external UI to show messages and prompts from
--cmd and buffer loading (e.g. swap files)

test/functional/helpers.lua

@@ -362,6 +362,8 @@ local function clear(...)
     table.insert(args, arg)
   end
   set_session(spawn(args, nil, env))
+  -- Dummy request so that --embed continues past UI initialization
+  session:request('nvim_eval', "0")
 end
 
 local function insert(...)