Example of a custom trace hook

[hotsphink]

I was lazy, and updated to -std=c++20 to use a designated initializer. It makes creating JSClassOps much quicker and nicer.

[hotsphink]

Er, I messed up and based this on my local custom.err branch. Which has now landed. I rebased it onto the landed esr115 tip and re-pushed. I don't know if I need to do anything else.

[ptomato]

No problem. If I was reviewing it I'd have one question about whether to use JS::GetMaybePtrFromReservedSlot() in CustomObject::ownedBox() and CustomObject::unownedBox(). On the one hand it seems like we don't need the isUndefined() check, but on the other hand we do check that the pointer isn't null before tracing.

[hotsphink]

I tried using JS::GetMaybePtrFromReservedSlot first, and got a compile error. But I just tried it again, and it worked fine. I must have done something wrong. (I think I misinterpreted the error to be saying that it needed my object type to inherit from JSObject, which isn't possible, but that's no different from any other API that takes JSObject*.)

I guess I'll make a PR for the change, and you can add any other comments there.

[hotsphink]

#84