patch 8.2.1613: Vim9: cannot pass "true" to prop_type_add()

Problem:    Vim9: cannot pass "true" to prop_type_add().
Solution:   Use tv_get_bool(). (closes #6850)
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index f771812..99ca31a 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -321,6 +321,17 @@
   bwipe!
 endfunc
 
+def Test_prop_add_vim9()
+  prop_type_add('comment', #{
+      highlight: 'Directory',
+      priority: 123,
+      start_incl: true,
+      end_incl: true,
+      combine: false,
+    })
+  prop_type_delete('comment')
+enddef
+
 def Test_prop_remove_vim9()
   new
   call AddPropTypes()