[css-scroll-anchoring-1] Make position: sticky an excluded subtree

`position: sticky` nodes aren't suitable anchor candidates as they may
become stuck in the future, which causes them to behave similarly to
`position: fixed`.

Author: eqrion

css-scroll-anchoring-1/Overview.bs

@@ -122,6 +122,7 @@ the implementation may wait until the anchor node is needed before computing it.
 
 	* |N|’s computed value of the 'display' property is ''display/none''.
 	* |N|’s computed value of the 'position' property is ''position/fixed''.
+	* |N|’s computed value of the 'position' property is ''position/sticky''.
 	* |N|’s computed value of the 'position' property is ''position/absolute''
 		and |N|’s <a>containing block</a> is an ancestor of the scrolling box.
 	* |N|’s computed value of the 'overflow-anchor' property is ''overflow-anchor/none''.