LibWeb: Add JSON serialization for nested browsing contexts

This changes allows for nested browser contexts to be embedded in the
serialized JSON of their container element (like `iframe`) and enables
their inspection in the DOM Inspector.

Userland/Libraries/LibWeb/Forward.h

@@ -116,6 +116,7 @@ class DOMRectReadOnly;
 
 namespace Web::HTML {
 class BrowsingContext;
+class BrowsingContextContainer;
 class CanvasRenderingContext2D;
 class CloseEvent;
 class DOMParser;