LibMedia+LibWeb: Implement media volume/muting

Author: https://github.com/Zaggy1024
Commit: 8d9a493b1b
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6410
Reviewed-by: https://github.com/R-Goc
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/HTML/HTMLMediaElement.h

@@ -211,6 +211,7 @@ private:
     void set_duration(double);
 
     void volume_or_muted_attribute_changed();
+    void update_volume();
 
     bool is_eligible_for_autoplay() const;
     bool has_ended_playback() const;