Add duplex property to Request

Oversight of #1457.

Tests: web-platform-tests/wpt#36051 & web-platform-tests/wpt#36048.

Closes #1486.

fetch.bs

@@ -6793,6 +6793,7 @@ interface Request {
   readonly attribute boolean isReloadNavigation;
   readonly attribute boolean isHistoryNavigation;
   readonly attribute AbortSignal signal;
+  readonly attribute RequestDuplex duplex;
 
   [NewObject] Request clone();
 };
@@ -6983,6 +6984,14 @@ object), initially null.
  {{AbortSignal}} object indicating whether or not <var>request</var> has been aborted, and its abort
  event handler.
 
+ <dt><code><var>request</var> . <a attribute for=Request>duplex</a></code>
+ <dd>Returns "<code>half</code>", meaning the fetch will be half-duplex (i.e., the user agent sends
+ the entire request before processing the response). In future, it could also return
+ "<code>full</code>", meaning the fetch will be full-duplex (i.e., the user agent can process the
+ response before sending the entire request) to indicate that the fetch will be full-duplex.
+ <span class=note>See <a href="https://github.com/whatwg/fetch/issues/1254">issue #1254</a> for
+ defining "<code>full</code>".</span>
+
  <dt><code><var>request</var> . <a method for=Request>clone</a>()</code>
  <dd><p>Returns a clone of <var>request</var>.
 </dl>
@@ -7433,6 +7442,9 @@ set; otherwise false.
 <p>The <dfn attribute for=Request><code>signal</code></dfn> getter steps are to return <a>this</a>'s
 <a for="Request">signal</a>.
 
+<p>The <dfn attribute for=Request><code>duplex</code></dfn> getter steps are to return
+"<code>half</code>".
+
 <hr>
 
 <p>The <dfn method for=Request><code>clone()</code></dfn> method steps are: