🏷️(type) add missing elements property in Plyr class

In Plyr class, you can access elements set in cache. This property is
missing in the class definition. The Plyr.Elements is for now
incomplete.

src/js/plyr.d.ts

@@ -138,6 +138,11 @@ declare class Plyr {
    */
   ratio?: string;
 
+  /**
+   * Access Elements cache
+   */
+  elements: Plyr.Elements;
+
   /**
    * Returns the current video Provider
    */
@@ -562,6 +567,27 @@ declare namespace Plyr {
     src?: string | string[];
   }
 
+  export interface Elements {
+    buttons: {
+      airplay?: HTMLButtonElement;
+      captions?: HTMLButtonElement;
+      download?: HTMLButtonElement;
+      fastForward?: HTMLButtonElement;
+      fullscreen?: HTMLButtonElement;
+      mute?: HTMLButtonElement;
+      pip?: HTMLButtonElement;
+      play?: HTMLButtonElement | HTMLButtonElement[];
+      restart?: HTMLButtonElement;
+      rewind?: HTMLButtonElement;
+      settings?: HTMLButtonElement;
+    };
+    captions: HTMLElement | null;
+    container: HTMLElement | null;
+    controls: HTMLElement | null;
+    fullscreen: HTMLElement | null;
+    wrapper: HTMLElement | null;
+  }
+
   interface SourceInfo {
     /**
      * Note: YouTube and Vimeo are currently not supported as audio sources.