[tz] Extra transition for Europe/London with 2023d

[Florian Weimer]

* Stephen Colebourne via tz:

> - getStandardOffset(Instant) - gets the standard ("winter") offset
> from UTC at an instant on the timeline

How does this work when the instant is using non-standard time?  The
standard offset is not defined at that time, is it?

Thanks,
Florian