BasemapGallery - Support auto-dismiss on selection

[dfeinzimer]

cc @rolson

[rolson]

I'd like to be able to do something like this:

        .sheet(isPresented: $isBasemapGalleryPresented) {
            BasemapGallery(portal: portal!, geoModel: map)
                .style(.grid(maxItemWidth: 100))
                .onBasemapSelection {
                    isBasemapGalleryPresented = false
                }
        }

Also, I am not sure the geoModel parameter makes sense as nullable currently without something like onBasemapSelection. Otherwise, how would a user make any use of the BasemapGallery?