LibWeb: Implement basic support for window[number]

This fixes an assertion on https://amazon.com/ since WindowProxy
would advertise "0" as an own property key, but then act like it was
a bogus property when actually queried for it directly.

Author: awesomekling

Tests/LibWeb/Text/expected/window-proxy-numeric-own-property.txt

@@ -0,0 +1,3 @@
+  true
+[object Window]
+false

Tests/LibWeb/Text/input/window-proxy-numeric-own-property.html

@@ -0,0 +1,9 @@
+<script src="include.js"></script>
+<iframe src="about:blank"></iframe>
+<script>
+    test(() => {
+        println(0 in window)
+        println(window[0])
+        println(1 in window)
+    });
+</script>

Userland/Libraries/LibWeb/HTML/WindowProxy.cpp

@@ -80,7 +80,12 @@ JS::ThrowCompletionOr<Optional<JS::PropertyDescriptor>> WindowProxy::internal_ge
 
         // 4. If maxProperties is greater than 0 and index is less than maxProperties, then set value to the WindowProxy object of the indexth document-tree child browsing context of W's browsing context, sorted in the order that their browsing context container elements were most recently inserted into W's associated Document, the WindowProxy object of the most recently inserted browsing context container's nested browsing context being last.
         if (max_properties > 0 && index < max_properties) {
-            // FIXME: Implement this.
+            JS::MarkedVector<BrowsingContext*> browsing_contexts { vm.heap() };
+            m_window->browsing_context()->for_each_child([&](BrowsingContext& child) {
+                if (child.container() && child.container()->in_a_document_tree())
+                    browsing_contexts.append(&child);
+            });
+            value = JS::Value(browsing_contexts[index]->window_proxy());
         }
 
         // 5. If value is undefined, then: