Reject private fields until --enable-private-fields shell option is removed

[arai-a]

private fields are enabled depending on the shell option, and the test is written based on the behavior.
we cannot turn on/off easily, so it's better just return NotImplemented

[Nicolas B. Pierron]

Could we use feature flags, which are toggle on/off by Smoosh?

[arai-a]

we can, but I'm not sure if it worth.

[Nicolas B. Pierron]

I agree, it might not be worth it for --enable-private-fields. However, it might be something to layout with option which would are more persistent.