LibWeb: Partially implement MediaSource.isTypeSupported()

Author: https://github.com/gmta
Commit: 24069d55bf
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2588
Reviewed-by: https://github.com/shannonbooth

Libraries/LibWeb/MediaSourceExtensions/MediaSource.h

@@ -30,6 +30,8 @@ public:
     void set_onsourceclose(GC::Ptr<WebIDL::CallbackType>);
     GC::Ptr<WebIDL::CallbackType> onsourceclose();
 
+    static bool is_type_supported(JS::VM&, String const&);
+
 protected:
     MediaSource(JS::Realm&);