Legend is missing for type "mesh3d" unless some surface trace is added

plotly.js added support for more trace legends but, for mesh3d, there is no legend below unless showlegend = TRUE for the mesh and a surface is added.

GH issue for more trace legends
merge for above

library(plotly)
library(rmarchingcubes)

R <- 2
f1 <- function(x, y, z){
  x^2 + y^2 + z^2 - R^2
}
f2 <- function(x, y, z){
  (x-2)^2 + (y-2)^2 + (z-2)^2 - R^2
}
g <- function(x, y, z) {
  pmax(f1(x, y, z), f2(x, y, z))
}

n <- 100
x_ <- y_ <- z_ <- seq(-R, R, length.out = n) 
Grid <- expand.grid(X = x_, Y = y_, Z = z_)
voxel <- with(Grid, array(g(X, Y, Z), dim = c(n, n, n)))
surf <- contour3d(voxel, level = 0, x_, y_, z_)
vertices <- surf$vertices
indices  <- surf$triangles - 1L

plot_ly(
  x = vertices[, 1], y = vertices[, 2], z = vertices[, 3],
  i = indices[, 1], j = indices[, 2], k = indices[, 3],
  type = "mesh3d",
  name = "hello world",
  showlegend = TRUE
) %>% layout(scene = list(aspectmode = "data"))

z <- c(
  c(8.83,8.89,8.81,8.87,8.9,8.87),
  c(8.89,8.94,8.85,8.94,8.96,8.92),
  c(8.84,8.9,8.82,8.92,8.93,8.91),
  c(8.79,8.85,8.79,8.9,8.94,8.92),
  c(8.79,8.88,8.81,8.9,8.95,8.92),
  c(8.8,8.82,8.78,8.91,8.94,8.92),
  c(8.75,8.78,8.77,8.91,8.95,8.92),
  c(8.8,8.8,8.77,8.91,8.95,8.94),
  c(8.74,8.81,8.76,8.93,8.98,8.99),
  c(8.89,8.99,8.92,9.1,9.13,9.11),
  c(8.97,8.97,8.91,9.09,9.11,9.11),
  c(9.04,9.08,9.05,9.25,9.28,9.27),
  c(9,9.01,9,9.2,9.23,9.2),
  c(8.99,8.99,8.98,9.18,9.2,9.19),
  c(8.93,8.97,8.97,9.18,9.2,9.18)
)
dim(z) <- c(15,6)

plot_ly(
  x = vertices[, 1], y = vertices[, 2], z = vertices[, 3],
  i = indices[, 1], j = indices[, 2], k = indices[, 3],
  type = "mesh3d",
  name = "hello",
  showscale = FALSE,
  showlegend = TRUE
) %>% 
  plotly::layout(scene = list(aspectmode = "data")) %>% 
  add_surface(z = ~z, name = "placeholder")

Start by reproducing the mesh3d examples in the issue and compare the behavior with the referenced plotly.js issue 2642 and pull request 4386. Trace how plotly.R passes mesh3d traces to plotly.js; done means a mesh3d trace shows its legend without requiring an added surface trace.