New spike defaults for v2

[nicolaskruchten]

I think the spikeline behaviour that we coerce today for x unified hovermode (in #4657) is better than the default behaviour... Should we change the compare defaults to be the same as the unified ones? i.e. *axis.spikesnap = 'hovered data' and spikedistance=-1 ?

[nicolaskruchten]

so the justification for these new defaults is basically the intuition that by default, spike lines should attach to what's being hovered on. hovered data doesn't force this, it seems to act to prevent spikelines a non-hovered point because it's geometrically closer (allowing for a fallback to a more-distant hover-winning point I guess). To guarantee that the hovered point is always in range, we set spikedistance to -1.

[nicolaskruchten]

Note that today, setting spikesnap='hovered data' forces spikes to true, which should no longer be the case when they're the default :)