fix(messages): unwanted ext_messages newlines for confirm() #38045

Problem:  Newlines emitted with ext_messages intended to position
          the message/prompt on the message grid.
Solution: Don't emit these newlines with ext_messages, followup to 4260f73e.

test/functional/ui/messages_spec.lua

@@ -51,7 +51,7 @@ describe('ui/ext_messages', function()
         {1:~                        }|*3
       ]],
       cmdline = { { content = { { '' } }, hl = 'MoreMsg', pos = 0, prompt = '[O]k: ' } },
-      messages = { { content = { { '\ntest\n', 6, 'MoreMsg' } }, kind = 'confirm' } },
+      messages = { { content = { { 'test', 6, 'MoreMsg' } }, kind = 'confirm' } },
     })
     feed('<cr>')
     screen:expect({