patch 8.2.2511: Vim9: cannot use Vim9 script syntax in some places

Problem:    Vim9: cannot use Vim9 script syntax in some places.
Solution:   Add the :vim9cmd command modifier. Incompatible: Makes ":vim9"
            mean ":vim9cmd" instead of ":vim9script".

src/testdir/test_vim9_cmd.vim

@@ -5,6 +5,16 @@ source vim9.vim
 source term_util.vim
 source view_util.vim
 
+def Test_vim9cmd()
+  var lines =<< trim END
+    vim9cmd var x = 123
+    let s:y = 'yes'
+    vim9c assert_equal(123, x)
+    vim9cm assert_equal('yes', y)
+  END
+  CheckScriptSuccess(lines)
+enddef
+
 def Test_edit_wildcards()
   var filename = 'Xtest'
   edit `=filename`