LibWeb: Stub out a basic IntersectionObserver interface

Note there are a couple of type differences between the spec and the IDL
file added in this commit. For example, we will need to support a type
of Variant to handle spec types such as "(double or sequence<double>)".
But for now, this allows web pages to construct an IntersectionObserver
with any valid type.

Userland/Libraries/LibWeb/Forward.h

@@ -204,6 +204,10 @@ namespace Web::HighResolutionTime {
 class Performance;
 }
 
+namespace Web::IntersectionObserver {
+class IntersectionObserver;
+}
+
 namespace Web::NavigationTiming {
 class PerformanceTiming;
 }
@@ -386,6 +390,7 @@ class HTMLUnknownElementWrapper;
 class HTMLVideoElementWrapper;
 class IdleDeadlineWrapper;
 class ImageDataWrapper;
+class IntersectionObserverWrapper;
 class KeyboardEventWrapper;
 class LocationObject;
 class MediaQueryListWrapper;