[css-view-transitions-2] Use readonly attributes for CSSViewTransitionRule (#10143)

Closes #10011

Author: noamr

css-view-transitions-2/Overview.bs

@@ -420,8 +420,8 @@ The {{CSSViewTransitionRule}} represents a ''@view-transition'' rule.
 
 		[Exposed=Window]
 		interface CSSViewTransitionRule : CSSRule {
-			attribute ViewTransitionNavigation navigation;
-			attribute CSSViewTransitionTypeSet types;
+			readonly attribute ViewTransitionNavigation navigation;
+			readonly attribute CSSViewTransitionTypeSet types;
 		};
 </xmp>