Enable layer visibility update from shiny

[anthonynorth]

A layer's visibility cannot be changed if the following conditions are met:

• updating layer via rdeck_proxy
• map layer_selector = TRUE and layer visibility_toggle = TRUE

https://github.com/anthonynorth/rdeck/blob/master/widget/widget.tsx#L49-L50

This constraint exists because add_x_layer() updates all of a layer's props, but the layer's visibility cannot (currently) be known in shiny. Updating a layer's visibility every time a reactive changes would be an irritating user experience.

Proposed change: visible = NULL to use the state in the browser, TRUE / FALSE to change it.

Example:

observe({
  rdeck_proxy("map") |>
    add_h3_hexagon_layer(
      id = "layer_id",
      data = some_new_data(),
      # client = truth
      visible = NULL,
    )
})