dom: Set "composed" flag on construction for some event types (#36239)

"Composed" flag (https://dom.spec.whatwg.org/#composed-flag) should be
properly set on event construction phase from optional "EventInit"
dictionary
(https://dom.spec.whatwg.org/#dom-eventinit-composed).

The limited set of event types (Custom/Error/Focus/Mouse) will be
affected by this CL (used in WPT tests).

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

- [X] There are WPT shadow-dom tests which have new issues related to
event "relatedTarget" property
tests/wpt/tests/shadow-dom/event-composed-path-with-related-target.html
  tests/wpt/tests/shadow-dom/event-with-related-target.html

Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>

components/script/dom/errorevent.rs

@@ -142,6 +142,7 @@ impl ErrorEventMethods<crate::DomTypeHolder> for ErrorEvent {
             init.error.handle(),
             can_gc,
         );
+        event.upcast::<Event>().set_composed(init.parent.composed);
         Ok(event)
     }