patch 8.2.2138: Vim9: "exit_cb" causes Vim to exit

Problem:    Vim9: "exit_cb" causes Vim to exit.
Solution:   Require white space after a command in Vim9 script. (closes #7467)
            Also fix that Vim9 style heredoc was not always recognized.

src/testdir/test_vim9_script.vim

@@ -3058,7 +3058,7 @@ def Test_put_with_linebreak()
   new
   var lines =<< trim END
     vim9script
-    pu=split('abc', '\zs')
+    pu =split('abc', '\zs')
             ->join()
   END
   CheckScriptSuccess(lines)
@@ -3079,6 +3079,13 @@ def Test_invoke_normal_in_visual_mode()
   xunmap <F3>
 enddef
 
+def Test_white_space_after_command()
+  var lines =<< trim END
+    exit_cb: Func})
+  END
+  CheckDefAndScriptFailure(lines, 'E1144:', 1)
+enddef
+
 " Keep this last, it messes up highlighting.
 def Test_substitute_cmd()
   new