crosstalk should support pointNumbers in event data

[cpsievert]

Some plotly.js events emit pointNumbers instead of pointNumber. I think this field is used to indicate an event has derived from a single graphical mark which represents numerous data points, for example clicking on a histogram bar:

https://codepen.io/cpsievert/pen/qoNPdx

Supporting this will greatly improve our ability to use aggregated traces as the source of the selection. Here's a minimal example:

library(plotly)
library(crosstalk)

m <- SharedData$new(mtcars)

subplot(
  plot_ly(m, x = ~factor(vs)),
  plot_ly(m, x = ~wt, y = ~mpg)
)