Add explicit [Exposed] (#935)

Author: saschanaz

worklets/Overview.bs

@@ -334,6 +334,7 @@ The {{Worklet}} object provides the capability to add module scripts into its as
 {{WorkletGlobalScope}}s and invoke their methods.
 
 <pre class='idl'>
+[Exposed=Window]
 interface Worklet {
     [NewObject] Promise&lt;void> addModule(USVString moduleURL, optional WorkletOptions options);
 };