Fallback choices become visible after reloading the story state

Someone reported this issue a few years ago
https://github.com/y-lohse/inkjs/issues/912

It has been fixed today by this pull-request
https://github.com/y-lohse/inkjs/pull/1130

Looking at the [relevant C# code](https://github.com/inkle/ink/blob/f4102e90ddf85ca86192d5b0c416060dfdfceec6/ink-engine-runtime/JsonSerialisation.cs#L595), I see that `isInvisibleDefault` is not serialized either in here. So the behaviour must be identical and the same issue must happen.

I am not able to test it myself (don't have a C# env available atm), but copying the changes from the inkjs over should be fairly straightforward ! (@tvararu even included a test case that could be added in here if need be)

Start with ink-engine-runtime/JsonSerialisation.cs and compare the relevant C# code with the fix in inkjs issue #912 and pull request #1130. Port the corresponding serialization change and add the test case mentioned from that pull request; done means fallback choices remain invisible after reloading story state.

Written by the indexing model from the issue text.