fix(messages): proper multiline Lua print() messages #31205

Problem:  Separate message emitted for each newline present in Lua
          print() arguments.
Solution: Make msg_multiline() handle NUL bytes. Refactor print() to use
          msg_multiline(). Refactor vim.print() to use print().

test/functional/lua/ui_event_spec.lua

@@ -142,7 +142,7 @@ describe('vim.ui_attach', function()
         'msg_history_show',
         {
           { 'echomsg', { { 0, 'message1', 0 } } },
-          { '', { { 0, 'message2', 0 } } },
+          { 'lua_print', { { 0, 'message2', 0 } } },
           { 'echomsg', { { 0, 'message3', 0 } } },
         },
       },