[d3-sankey] Missing Iterations (#16916)

* [Fix] Adds missing `iterations` setter and getter for `SankeyLayout`

types/d3-sankey/d3-sankey-tests.ts

@@ -162,6 +162,19 @@ slgDAG = slgDAG.size([1200, 800]);
 
 size = slgDAG.size();
 
+// ---------------------------------------------------------------------------
+// Iterations
+// ---------------------------------------------------------------------------
+
+// Set -----------------------------------------------------------------------
+
+// test return type for chainability
+slgDAG = slgDAG.iterations(40);
+
+// Get -----------------------------------------------------------------------
+
+num = slgDAG.iterations();
+
 // ---------------------------------------------------------------------------
 // Nodes
 // ---------------------------------------------------------------------------

types/d3-sankey/index.d.ts

@@ -279,6 +279,17 @@ export interface SankeyLayout<Data, N extends SankeyExtraProperties, L extends S
      * @param size A two element array of [width, height] in pixels which defaults to [1, 1].
      */
     size(size: [number, number]): this;
+
+    /**
+     * Return the current number of relaxation iterations, which defaults to 32.
+     */
+    iterations(): number;
+    /**
+     * Set the number of relaxation iterations when generating the layout and return this Sankey layout generator.
+     *
+     * @param iterations Number of relaxation iterations, which defaluts to 32.
+     */
+    iterations(iterations: number): this;
 }
 
 /**