Have add_splom() route all numeric columns to dimensions

So that something like

plot_ly(mpg) %>%
  add_splom()

Would do:

plot_ly(
    mpg,
    type = 'splom',
    dimensions = list(
      list(values = ~displ, label = "displ"),
      list(values = ~year, label = "year"),
      list(values = ~cyl, label = "cyl"),
      list(values = ~cty, label = "cty"),
      list(values = ~hwy, label = "hwy")
    )
  )

从 add_splom() 入口点开始，将其行为与提供的 plot_ly(mpg) 示例和显式维度示例进行比较。当 add_splom() 将数据中的每个数值列都路由到一个与所示输出等效的带标签维度时，即视为完成。