style: Implement overflow: overlay as an alias on auto, and enable on Nightly

See related webcompat issues and discussion in
https://github.com/w3c/csswg-drafts/issues/8063

Differential Revision: https://phabricator.services.mozilla.com/D165058

components/style/values/specified/box.rs

@@ -2207,7 +2207,10 @@ impl Parse for Overflow {
             "clip" => Self::Clip,
             #[cfg(feature = "gecko")]
             "-moz-hidden-unscrollable" if static_prefs::pref!("layout.css.overflow-moz-hidden-unscrollable.enabled") => {
-               Overflow::Clip
+                Overflow::Clip
+            },
+            "overlay" if static_prefs::pref!("layout.css.overflow-overlay.enabled") => {
+                Overflow::Auto
             },
         })
     }