Add console.timeStamp() to the specification

This adds a minimal (vague) definition for the `timeStamp()` method,
based on its current behavior across Chromium, Firefox, and Safari,
and the MDN documentation[^1].

[^1]: https://developer.mozilla.org/en-US/docs/Web/API/console/timestamp_static

Ref: #140

index.bs

@@ -66,6 +66,7 @@ namespace console { // but see namespace object requirements below
   undefined time(optional DOMString label = "default");
   undefined timeLog(optional DOMString label = "default", any... data);
   undefined timeEnd(optional DOMString label = "default");
+  undefined timeStamp(optional DOMString label = "default");
 };
 </pre>
 
@@ -274,6 +275,12 @@ for plans to make {{console/timeEnd()}} and {{console/timeLog()}} formally repor
 console when a given |label| does not exist in the associated <a>timer table</a>.
 </p>
 
+<h4 id="timestamp" oldids="timestamp-label,dom-console-timestamp" method for="console">timeStamp(|label|)</h4>
+
+1. If the developer is recording a performance trace, add a single marker to the performance trace with the |label|.
+1. Otherwise, do nothing.
+1. Return *undefined*.
+
 <h2 id="supporting-ops">Supporting abstract operations</h2>
 
 <h3 id="logger" abstract-op lt="Logger">Logger(|logLevel|, |args|)</h3>