Adding geom_GeomStepribbon()

I have a chart where I use geom_steppribbon() which is part of the pammtools package. This geom has yet to be implemented in plotly.
I am happy to help with the implementation. However, I have not yet contributed to plotly and would need some help where to get started.

This would be a minimal example of a chart with geom_stepribbon:

library(ggplot2)
library(pammtools)

huron <- data.frame(year = 1875:1972, level = as.vector(LakeHuron))

ggplot(huron, aes(year)) +
pammtools::geom_stepribbon(aes(ymin = level - 1, ymax = level + 1)) +
geom_step(aes(y = level))

Thank you already for your help.

Best,
Leo

Issue ではファイルやテストが指定されていないため、まず plotly.R にある geom_step レイヤーと ribbon レイヤーの既存の処理を見つけ、pammtools::geom_stepribbon の例と比較します。最小限の LakeHuron チャートが、要求されたステップリボン付きで plotly.R にレンダリングされれば完了です。