Restore hls.media prop and correct type.

I erroneously removed this property in my last PR, but it's been brought to my attention by another dev. `media` is unset at construction, but is set to an `HTMLMediaElement` when passed via `attachMedia` and cleared to `null` when `detachMedia` is called. The docs appear to imply that this property is meant to be readonly, and that altering it is to be done with the above referenced methods.

types/hls.js/index.d.ts

@@ -1666,6 +1666,16 @@ declare class Hls {
      *  default valus is hls.firstLevel
      */
     startLevel: number;
+    /**
+     * (default: true)
+     * if set to true, start level playlist and first fragments will be loaded automatically, after triggering of Hls.Events.MANIFEST_PARSED event
+     * if set to false, an explicit API call (hls.startLoad(startPosition=-1)) will be needed to start quality level/fragment loading.
+     */
+    autoStartLoad: boolean;
+    /**
+     * get: Return the bound videoElement from the hls instance
+     */
+    readonly media?: HTMLVideoElement | null;
     /**
      *  hls.js config
      */