editorial: Define a task source for the tasks run by this specification.

This allows implementations to better decide when to run events and tasks
from this spec in relation to others, and specifying a task source is a
requirement when queuing tasks like we do [1].

[1] https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-for-spec-authors

Rather than picking a generic task source from the HTML spec, we define a
new task source, the "screen wake lock task source", following the current
guidance in w3ctag/design-principles#38 (which is still open).

index.html

@@ -252,6 +252,10 @@ <h2>
       <h3>
         Concepts and state record
       </h3>
+      <p>
+        The <a>task source</a> for the <a>tasks</a> mentioned in this
+        specification is the <dfn>screen wake lock task source</dfn>.
+      </p>
       <p>
         The term <dfn>platform wake lock</dfn> refers to platform interfaces
         with which the user agent interacts to query state and acquire and