LibWeb: Use the specification defined types in IntersectionObserverInit

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.idl

@@ -1,3 +1,4 @@
+#import <DOM/Document.idl>
 #import <DOM/Element.idl>
 #import <DOM/Node.idl>
 
@@ -16,11 +17,7 @@ interface IntersectionObserver {
 };
 
 dictionary IntersectionObserverInit {
-    // FIXME: Should be: (Element or Document)?
-    Node? root = null;
-
+    (Element or Document)? root = null;
     DOMString rootMargin = "0px";
-
-    // FIXME: Should be: (double or sequence<double>)
-    any threshold = 0;
+    (double or sequence<double>) threshold = 0;
 };