LibWeb: Implement window.event as a [Replaceable] property

This is the result of debugging React DOM, which would throw a TypeError
when assigning to window.event in strict mode and then not complete
rendering - here:
cae6350/packages/shared/invokeGuardedCallbackImpl.js (L134)

With this change, the following minimal React example now works!

    <div id="app"></div>
    <script src="react.development.js"></script>
    <script src="react-dom.development.js"></script>
    <script>
        ReactDOM.render(
            React.createElement("h1", null, "Hello World"),
            document.getElementById("app")
        );
    </script>

Userland/Libraries/LibWeb/Bindings/WindowObject.h

@@ -68,6 +68,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(screen_getter);
 
     JS_DECLARE_NATIVE_FUNCTION(event_getter);
+    JS_DECLARE_NATIVE_FUNCTION(event_setter);
 
     JS_DECLARE_NATIVE_FUNCTION(inner_width_getter);
     JS_DECLARE_NATIVE_FUNCTION(inner_height_getter);