[scroll-animations-1] Define view timeline range 'scroll' (w3c#13306)

This adds a named ViewTimeline range of 'scroll' as resolved[1] on.

[1] w3c#9367 (comment)

Co-authored-by: David Awogbemila <awogbemila@chromium.org>

Author: DavMila

scroll-animations-1/Overview.bs

@@ -682,6 +682,15 @@ spec:selectors-4; type:dfn; text:selector
 			* 100% progress represents the earliest position at which
 				the [=end=] [=border edge=] of the element’s [=principal box=]
 				coincides with the [=start=] edge of its [=view progress visibility range=].
+
+		<dt><dfn>scroll</dfn>
+		<dd>
+			Represents the full range of the [=scroll container=] on which the [=view progress timeline=]
+			is defined.
+			* 0% progress represents the startmost scroll position of the [=scroll container=]
+				underlying the [=view progress timeline=].
+			* 100% progress represents the endmost scroll position of the [=scroll container=]
+				underlying the [=view progress timeline=].
 	</dl>
 
 	ISSUE: Insert diagrams.