Add shadowPane option to Marker (Leaflet#6460)

* Add shadowPane option to Marker

Add shadowPane option to Marker, so developers can specify pane for marker's shadow as suggested in Leaflet#6447

* Update Marker.js

src/layer/marker/Marker.js

@@ -64,6 +64,10 @@ export var Marker = Layer.extend({
 		// `Map pane` where the markers icon will be added.
 		pane: 'markerPane',
 
+		// @option pane: String = 'shadowPane'
+		// `Map pane` where the markers shadow will be added.
+		shadowPane: 'shadowPane',
+
 		// @option bubblingMouseEvents: Boolean = false
 		// When `true`, a mouse event on this marker will trigger the same event on the map
 		// (unless [`L.DomEvent.stopPropagation`](#domevent-stoppropagation) is used).
@@ -251,7 +255,7 @@ export var Marker = Layer.extend({
 		}
 		this._initInteraction();
 		if (newShadow && addShadow) {
-			this.getPane('shadowPane').appendChild(this._shadow);
+			this.getPane(options.shadowPane).appendChild(this._shadow);
 		}
 	},