♻️(type) use Plyr.Provider for the readonly provider property

A type exists to define all available providers. This type isn't used in
the Plyr class definition and the same provider list is also defined.
This code is refactored to use the Plyr.Provider type

src/js/plyr.d.ts

@@ -141,7 +141,7 @@ declare class Plyr {
   /**
    * Returns the current video Provider
    */
-  readonly provider: 'html5' | 'vimeo' | 'youtube';
+  readonly provider: Plyr.Provider;
 
   /**
    * Returns the native API for Vimeo or Youtube players