Added control/layers

[legokichi]

• https://leafletjs.com/reference.html#control-layers

usage

let opt = &leaflet::TileLayerOptions::new();
opt.set_attribution(r#"© <a href="http://www.openstreetmap.org/copyright">OpenStreetMap</a>"#.to_string());
let osm = TileLayer::new_options("https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png", opt);
gsi.add_to(&map);
let opt = js_sys::Object::new();
js_sys::Reflect::set(&opt, &JsValue::from("OpenStreetMap"), &JsValue::from(osm)).unwrap();
let control = leaflet::LayersControl::new(&opt);
control.add_to(&map);