LibWeb: Make “create an event” set the event’s isTrusted to true

This change ensures that when then the code corresponding to the “create
an event” operation at https://dom.spec.whatwg.org/#concept-event-create
is called, the event’s isTrusted is set to true — as the spec requires.

That causes the failures for the following WPT tests to pass:

- https://wpt.fyi/results/html/semantics/forms/the-input-element/checkbox.html?run_id=5080423051034624
- https://wpt.fyi/results/html/semantics/interactive-elements/the-dialog-element/dialog-close.html?run_id=5080423051034624

…and there are likely a number of similar WPT tests that hit this same
code path which this commit will cause to be changed to passes.

Otherwise, without this change, the “create event” implementation
doesn’t conform to the spec requirements – nor behave interoperably with
other existing engines — and those WPT test would continue to fail.

This change also ensures that isTrusted continues to be set to false for
synthetic events.

Tests/LibWeb/Text/expected/synthetic-event-constructor-istrusted-check.txt

@@ -0,0 +1 @@
+new Event('x').isTrusted is false

Tests/LibWeb/Text/input/synthetic-event-constructor-istrusted-check.html

@@ -0,0 +1,6 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        println(`new Event('x').isTrusted is ${new Event('x').isTrusted}`);
+    });
+</script>

Userland/Libraries/LibWeb/DOM/Event.cpp

@@ -18,14 +18,18 @@ namespace Web::DOM {
 
 JS_DEFINE_ALLOCATOR(Event);
 
+// https://dom.spec.whatwg.org/#concept-event-create
 JS::NonnullGCPtr<Event> Event::create(JS::Realm& realm, FlyString const& event_name, EventInit const& event_init)
 {
-    return realm.heap().allocate<Event>(realm, realm, event_name, event_init);
+    auto event = realm.heap().allocate<Event>(realm, realm, event_name, event_init);
+    // 4. Initialize event’s isTrusted attribute to true.
+    event->m_is_trusted = true;
+    return event;
 }
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<Event>> Event::construct_impl(JS::Realm& realm, FlyString const& event_name, EventInit const& event_init)
 {
-    return create(realm, event_name, event_init);
+    return realm.heap().allocate<Event>(realm, realm, event_name, event_init);
 }
 
 // https://dom.spec.whatwg.org/#inner-event-creation-steps