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/IntersectionObserver/IntersectionObserver.cpp

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/IntersectionObserver/IntersectionObserver.h>
+
+namespace Web::IntersectionObserver {
+
+// https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-intersectionobserver
+NonnullRefPtr<IntersectionObserver> IntersectionObserver::create_with_global_object(JS::GlobalObject& global_object, JS::Value callback, IntersectionObserverInit const& options)
+{
+    // FIXME: Implement
+    (void)global_object;
+    (void)callback;
+    (void)options;
+
+    return adopt_ref(*new IntersectionObserver);
+}
+
+// https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-observe
+void IntersectionObserver::observe(DOM::Element& target)
+{
+    // FIXME: Implement
+    (void)target;
+}
+
+// https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-unobserve
+void IntersectionObserver::unobserve(DOM::Element& target)
+{
+    // FIXME: Implement
+    (void)target;
+}
+
+// https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-disconnect
+void IntersectionObserver::disconnect()
+{
+    // FIXME: Implement
+}
+
+}