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_assign.vim

@@ -982,6 +982,17 @@ def Test_heredoc()
         var&lines =<< trim END
         x
         x
+      enddef
+      defcompile
+  [END]
+  CheckScriptFailure(lines, 'E1145: Missing heredoc end marker: END')
+  delfunc! g:Func
+
+  lines =<< trim [END]
+      def Func()
+        var lines =<< trim END
+        x
+        x
         x
         x
         x
@@ -991,7 +1002,7 @@ def Test_heredoc()
       enddef
       call Func()
   [END]
-  CheckScriptFailure(lines, 'E990:')
+  CheckScriptFailure(lines, 'E1145: Missing heredoc end marker: END')
   delfunc! g:Func
 enddef