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

该 issue 没有指出任何文件或测试；先定位 plotly.R 中对 geom_step 和 ribbon 图层的现有处理，然后将其与 pammtools::geom_stepribbon 示例进行比较。完成的标准是：最小的 LakeHuron 图表能够在 plotly.R 中以所要求的阶梯带进行渲染。