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/core/startup_spec.lua

@@ -277,10 +277,8 @@ describe('startup', function()
 
       -- nvim <vim args> -l foo.lua <vim args>
       assert_l_out(
-        -- luacheck: ignore 611 (Line contains only whitespaces)
         [[
             wrap
-          
           bufs:
           nvim args: 7
           lua args: { "-c", "set wrap?",