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
+}
+
+}

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web::IntersectionObserver {
+
+struct IntersectionObserverInit {
+    DOM::Node* root { nullptr };
+    String root_margin { "0px"sv };
+    JS::Value threshold { 0 };
+};
+
+// https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
+class IntersectionObserver
+    : public RefCounted<IntersectionObserver>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::IntersectionObserverWrapper;
+
+    static NonnullRefPtr<IntersectionObserver> create_with_global_object(JS::GlobalObject&, JS::Value callback, IntersectionObserverInit const& options = {});
+
+    void observe(DOM::Element& target);
+    void unobserve(DOM::Element& target);
+    void disconnect();
+};
+
+}

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.idl

@@ -0,0 +1,22 @@
+[Exposed=(Window)]
+interface IntersectionObserver {
+    // FIXME: Should be: IntersectionObserverCallback
+    constructor(any callback, optional IntersectionObserverInit options = {});
+
+    undefined observe(Element target);
+    undefined unobserve(Element target);
+    undefined disconnect();
+
+    // FIXME:
+    // sequence<IntersectionObserverEntry> takeRecords();
+};
+
+dictionary IntersectionObserverInit {
+    // FIXME: Should be: (Element or Document)?
+    Node? root = null;
+
+    DOMString rootMargin = "0px";
+
+    // FIXME: Should be: (double or sequence<double>)
+    any threshold = 0;
+};