LibWeb: Implement basic version of CSSOM View's VisualViewport

We got some errors while loading https://twinings.co.uk/ about this
interface missing, and it looked fairly simple so I sketched it out.
Note that I did leave some FIXMEs where it's not clear exactly which
metrics we should be returning.

Userland/Libraries/LibWeb/HTML/Window.idl

@@ -1,6 +1,7 @@
 #import <Crypto/Crypto.idl>
 #import <CSS/MediaQueryList.idl>
 #import <CSS/Screen.idl>
+#import <CSS/VisualViewport.idl>
 #import <DOM/Document.idl>
 #import <DOM/EventHandler.idl>
 #import <DOM/EventTarget.idl>
@@ -57,6 +58,7 @@ interface Window : EventTarget {
     // https://w3c.github.io/csswg-drafts/cssom-view/#extensions-to-the-window-interface
     [NewObject] MediaQueryList matchMedia(CSSOMString query);
     [SameObject, Replaceable] readonly attribute Screen screen;
+    [SameObject, Replaceable] readonly attribute VisualViewport? visualViewport;
 
     // viewport
     [Replaceable] readonly attribute long innerWidth;