Add spikelines to ternary plots

[Lexachoc]

Closes #7972

Demo in #7972 (comment)

Similar to Cartesian spikelines, ternary axes support the existing spikeline attributes:

• showspikes
• spikecolor
• spikethickness
• spikedash
• spikemode
• spikesnap

The ternary spikelines also respect spikedistance.

Note: This implementation always displays the corresponding axis value labels (commonlabel) together with the spikelines.

In Cartesian plots, these labels are controlled by hovermode. Ternary plots currently do not have a hovermode option that fits naturally to ternary vertices, and I prefer to always show the labels together with the ternary spikelines instead, which also seems more intuitive for ternary plots. And these labels help the user to quickly understand the axis values.

The axis value labels also respect the axis attribute: hoverformat, so they use the same numeric formatting consistent with the ternary hoverlabel.

[Lexachoc]

Sorry about the force-push! I noticed afterward that the guideline ask not to force-push open PR branches.

[camdecoster]

Thanks for the PR! Our team will take a look and provide some feedback. No worries about the force push; it's mostly important for after a review starts.

[Lexachoc]

While testing scatterternarygl (for issue #7058, PR coming soon), I found that the normal hover search already returns a usable point with a finite spikeDistance, but the input pointData.spikeDistance remains Infinity because scattergl returns a new hover point object.

This caused the current ternary spike logic to run a redundant second hoverPoints() search. The next commit will reuse the normal hover result when available and only performs the fallback search when necessary, e.g. when the point is outside hoverdistance but still within spikedistance.

Below is a recording showing the spikelines feature working with a scatterternarygl trace. They work together smoothly without noticeable lag.

spikelines.with.scatterternarygl.trace.mp4