LibWeb: Fire MediaQueryListEvents when an MQL's match-state changes

The HTML event loop does a check for MQL match-state changes and
dispatches the events. This requires us to keep a list of MQLs on the
Document.

Userland/Libraries/LibWeb/Forward.h

@@ -31,6 +31,7 @@ class Length;
 class MediaList;
 class MediaQuery;
 class MediaQueryList;
+class MediaQueryListEvent;
 class PropertyOwningCSSStyleDeclaration;
 class Screen;
 class Selector;
@@ -375,6 +376,7 @@ class ImageDataWrapper;
 class KeyboardEventWrapper;
 class LocationObject;
 class MediaQueryListWrapper;
+class MediaQueryListEventWrapper;
 class MessageChannelWrapper;
 class MessageEventWrapper;
 class MessagePortWrapper;