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.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;
+};