feat(slides): add method to get previous index

Slides added method wrapper: `getPreviousIndex()`

references #5508

ionic/components/slides/slides.ts

@@ -827,6 +827,15 @@ export class Slides extends Ion {
     return this.slider.activeIndex;
   }
 
+  /**
+   * Get the index of the previous slide.
+   *
+   * @returns {number} The index number of the previous slide.
+   */
+  getPreviousIndex(): number {
+    return this.slider.previousIndex;
+  }
+
   /**
    * Get the total number of slides.
    *