Support grid::pattern() / gradient fills

[trevorld]

{ggplot2} 3.5.0 geoms were updated to accept grid.pattern() / gradient fills. The geoms in {ggridges} could also be updated to accept such fills:

• Bump required ggplot2 version number to v3.5
• Replace code like fill = alpha(data$fill, data$alpha) with fill = ggplot2::fill_alpha(data$fill, data$alpha)

[clauswilke]

You're welcome to prepare a PR.