events: change status of event.returnvalue to legacy

`event.returnvalue` is described as legacy in spec.
Plus, add missed '#'(private member) of defaultPrevented
in implementation.

Refs: https://dom.spec.whatwg.org/#interface-event
Refs: https://developer.mozilla.org/en-US/docs/Web/API/Event/returnValue
PR-URL: #46175
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: deokjinkim

doc/api/events.md

@@ -2111,8 +2111,11 @@ Sets the `defaultPrevented` property to `true` if `cancelable` is `true`.
 added: v14.5.0
 -->
 
+> Stability: 3 - Legacy: Use [`event.defaultPrevented`][] instead.
+
 * Type: {boolean} True if the event has not been canceled.
 
+The value of `event.returnValue` is always the opposite of `event.defaultPrevented`.
 This is not used in Node.js and is provided purely for completeness.
 
 #### `event.srcElement`
@@ -2429,6 +2432,7 @@ to the `EventTarget`.
 [`emitter.listenerCount()`]: #emitterlistenercounteventname
 [`emitter.removeListener()`]: #emitterremovelistenereventname-listener
 [`emitter.setMaxListeners(n)`]: #emittersetmaxlistenersn
+[`event.defaultPrevented`]: #eventdefaultprevented
 [`event.stopPropagation()`]: #eventstoppropagation
 [`event.target`]: #eventtarget
 [`events.defaultMaxListeners`]: #eventsdefaultmaxlisteners

lib/internal/event_target.js

@@ -236,7 +236,7 @@ class Event {
   get returnValue() {
     if (!isEvent(this))
       throw new ERR_INVALID_THIS('Event');
-    return !this.defaultPrevented;
+    return !this.#cancelable || !this.#defaultPrevented;
   }
 
   /**