Add [Unscopable] to Document.prototype.fullscreen

This appears to be needed per
https://bugzilla.mozilla.org/show_bug.cgi?id=1364025.

fullscreen.bs

@@ -144,7 +144,7 @@ partial interface Element {
 
 partial interface Document {
   [LenientSetter] readonly attribute boolean fullscreenEnabled;
-  [LenientSetter] readonly attribute boolean fullscreen; // historical
+  [LenientSetter, Unscopable] readonly attribute boolean fullscreen; // historical
 
   Promise<void> exitFullscreen();