docs(actor2d): add typescript definitions for [set/get]LayerNumber in vtkActor2D

Author: sankhesh

Sources/Rendering/Core/Actor2D/index.d.ts

@@ -54,6 +54,28 @@ export interface vtkActor2D extends vtkProp {
    */
   getMapper(): vtkMapper2D;
 
+  /**
+   * Set the layer number for this 2D actor.
+   * The scenegraph uses this layer number to sort actor 2D overlays/underlays on top of each other.
+   * The actor2D with the highest layer number is going to be rendered at the very front i.e. it is
+   * the top-most layer.
+   * If two actor2D instances share the same layer number, they are rendered in the order in which
+   * they were added to the renderer via `addActor` or `addActor2D`.
+   * By default, each actor2D has a layer number of 0.
+   */
+  setLayerNumber(layer: number): void;
+
+  /**
+   * Get the layer number for this 2D actor.
+   * The scenegraph uses this layer number to sort actor 2D overlays/underlays on top of each other.
+   * The actor2D with the highest layer number is going to be rendered at the very front i.e. it is
+   * the top-most layer.
+   * If two actor2D instances share the same layer number, they are rendered in the order in which
+   * they were added to the renderer via `addActor` or `addActor2D`.
+   * By default, each actor2D has a layer number of 0.
+   */
+  getLayerNumber(): number;
+
   /**
    *
    */