Add feature check to doctests

[yrns]

I noticed many doctests fail since JanetClient isn't in the default feature set. The doctests can be wrapped with #[cfg_attr(..., doc = "...")] to skip them with the default features. I can do the rest if you think this is worth the effort (there's a lot). Or, just close if not.

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[GrayJack]

It is not as syntactically pleasing as inline docs and we would lose the example when the features are not set, but it can be very useful.

Another two options that passed my mind are:

1. to apply ignore after the code block initializer ("```") when the features needed for the client are not set or
2. make the amalgation feature default.

The first one I have no idea how feasible it is to do.

The second will make it more bothersome for users that want to write a native lib for Janet and people using the system Janet, so I'm not sure if this is a good path

So, unless the first is feasible and easy to do, I think this is worth the effort.

I'll do some more digging today how feasible option 1 is

[yrns]

Is this better or worse? Not sure. It works, at least.

    #[cfg_attr(any(feature = "amalgation", feature = "link-system"), doc = "```")]
    #[cfg_attr(
        not(any(feature = "amalgation", feature = "link-system")),
        doc = "```ignore"
    )]

2. make the `amalgation` feature default.

Users can always disable default features. Maybe this is best?