Make event handlers share a code path with addEventListener()

This hook was provided whatwg/dom#596 and event handlers need to use it to fix whatwg/dom#365 and w3c/ServiceWorker#1004 properly. Tests will be added as part of resolving those issues.

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@@ -3183,10 +3183,13 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#composed-flag">composed flag</dfn></li>
      <li><dfn data-x="dom-Event-isTrusted" data-x-href="https://dom.spec.whatwg.org/#dom-event-istrusted"><code>isTrusted</code></dfn> attribute</li>
      <li><dfn data-x="dom-Event-initEvent" data-x-href="https://dom.spec.whatwg.org/#dom-event-initevent"><code>initEvent()</code></dfn> method</li>
+     <li><dfn data-x-href="https://dom.spec.whatwg.org/#add-an-event-listener">add an event listener</dfn></li>
      <li><dfn data-x="dom-EventTarget-addEventListener" data-x-href="https://dom.spec.whatwg.org/#dom-eventtarget-addeventlistener"><code>addEventListener()</code></dfn> method</li>
      <li><dfn data-x="dom-EventListener" data-x-href="https://dom.spec.whatwg.org/#callbackdef-eventlistener"><code>EventListener</code></dfn> callback interface</li>
      <li>The <dfn data-x="concept-event-type" data-x-href="https://dom.spec.whatwg.org/#dom-event-type">type</dfn> of an event</li>
-     <li>The concept of an <dfn data-x="concept-event-listener" data-x-href="https://dom.spec.whatwg.org/#concept-event-listener">event listener</dfn> and the <span data-x="concept-event-listener">event listeners</span> associated with an <code>EventTarget</code></li>
+     <li>An <dfn data-x-href="https://dom.spec.whatwg.org/#concept-event-listener">event listener</dfn> and its
+            <dfn data-x="event listener type" data-x-href="https://dom.spec.whatwg.org/#event-listener-type">type</dfn> and
+            <dfn data-x="event listener callback" data-x-href="https://dom.spec.whatwg.org/#event-listener-callback">callback</dfn></li>
 
      <li>The <dfn data-x="document's character encoding" data-x-href="https://dom.spec.whatwg.org/#concept-document-encoding">encoding</dfn> (herein the <i>character encoding</i>) and
              <dfn data-x="concept-document-content-type" data-x-href="https://dom.spec.whatwg.org/#concept-document-content-type">content type</dfn> of a <code>Document</code></li>
@@ -89690,22 +89693,38 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
   <hr>
 
-  <p>When an <span data-x="event handlers">event handler</span> <var>H</var> of an element or object
-  <var>T</var> implementing the <code>EventTarget</code> interface is first set to a non-null value,
-  the user agent must append an <span data-x="concept-event-listener">event listener</span> to the
-  list of <span data-x="concept-event-listener">event listeners</span> associated with <var>T</var>
-  with <b>type</b> set to the <dfn data-export="">event handler event type</dfn> corresponding to
-  <var>H</var> and <b>callback</b> set to the result of creating a Web IDL <code
-  data-x="dom-EventListener">EventListener</code> instance representing a reference to a function of
-  one argument that executes the steps of <span>the event handler processing algorithm</span>, given
-  <var>H</var> and its argument. The <code data-x="dom-EventListener">EventListener</code>'s
-  <span>callback context</span> can be arbitrary; it does not impact the steps of <span>the event
-  handler processing algorithm</span>. <ref spec=DOM></p>
-
-  <p class="note">The <b>callback</b> is emphatically <em>not</em> the <span data-x="event
-  handlers">event handler</span> itself. Every event handler ends up registering the same
-  <b>callback</b>, the algorithm defined below, which takes care of invoking the right callback, and
-  processing the callback's return value.</p>
+  <p>When an <span data-x="event handlers">event handler</span> <var>H</var> of an
+  <code>EventTarget</code> object <var>T</var> is first set to a non-null value, then:
+
+  <ol>
+   <li>
+    <p>Let <var>callback</var> be the result of creating a Web IDL <code
+    data-x="dom-EventListener">EventListener</code> instance representing a reference to a function
+    of one argument that executes the steps of <span>the event handler processing algorithm</span>,
+    given <var>H</var> and its argument.</p>
+
+    <p>The <code data-x="dom-EventListener">EventListener</code>'s <span>callback context</span> can
+    be arbitrary; it does not impact the steps of <span>the event handler processing
+    algorithm</span>. <ref spec=DOM></p>
+
+    <p class="note">The callback is emphatically <em>not</em> the <span data-x="event
+    handlers">event handler</span> itself. Every event handler ends up registering the same
+    callback, the algorithm defined below, which takes care of invoking the right callback, and
+    processing the callback's return value.</p>
+   </li>
+
+   <li>
+    <p>Let <var>listener</var> be a new <span>event listener</span> whose <span data-x="event
+    listener type">type</span> is the <dfn data-export="">event handler event type</dfn>
+    corresponding to <var>H</var> and <span data-x="event listener callback">callback</span> is
+    <var>callback</var>.</p>
+
+    <p class="note">To be clear, an <span>event listener</span> is different from an <code
+    data-x="dom-EventListener">EventListener</code>.</p>
+   </li>
+
+   <li><p><span>Add an event listener</span> with <var>T</var> and <var>listener</var>.</p></li>
+  </ol>
 
   <p class="note">This only happens the first time the <span data-x="event handlers">event
   handler</span>'s value is set. Since listeners are called in the order they were registered, the
@@ -120803,6 +120822,7 @@ INSERT INTERFACES HERE
   Carlos Gabriel Cardona,
   Carlos Perell&oacute; Mar&iacute;n,
   Casey Leask,
+  Cătălin Badea,
   C&#x103;t&#x103;lin Mari&#x219;,
   Chao Cai,
   &#xc724;&#xc11d;&#xcc2c; (Channy Yun),