[Editorial] Add "noexport" to :past and :future dfns (#510)

The `:past` and `:future` pseudo-classes are formally defined in Selectors 4,
which the WebVTT spec correctly references. It is fine to re-define these
terms locally. The problem is that previous commit flagged these definitions
as being selectors, which is correct but also made Bikeshed export them.

In other words, these definitions now conflict with those in Selectors 4.

This update flags these definitions with a `noexport` attribute to tell
Bikeshed to keep these definitions local.

index.bs

@@ -5480,7 +5480,7 @@ would have been applied to the <a>list of WebVTT Node Objects</a>, must instead
 <p>The '':past'' and '':future'' pseudo-classes sometimes match <a lt="WebVTT Node Object">WebVTT
 Node Objects</a>. [[!SELECTORS4]]</p>
 
-<p>The <dfn selector>:past</dfn> pseudo-class only matches <a lt="WebVTT Node Object">WebVTT Node Objects</a>
+<p>The <dfn selector noexport>:past</dfn> pseudo-class only matches <a lt="WebVTT Node Object">WebVTT Node Objects</a>
 that are <i>in the past</i>.</p>
 
 <p algorithm="in the past">A <a>WebVTT Node Object</a> |c| is <dfn abstract-op>in the past</dfn> if, in a
@@ -5489,7 +5489,7 @@ there exists a <a>WebVTT Timestamp Object</a> whose value is less than the <a>cu
 position</a> of the <a>media element</a> that is the <i>matched element</i>, entirely after the
 <a>WebVTT Node Object</a> |c|.</p>
 
-<p>The <dfn selector>:future</dfn> pseudo-class only matches <a lt="WebVTT Node Object">WebVTT Node
+<p>The <dfn selector noexport>:future</dfn> pseudo-class only matches <a lt="WebVTT Node Object">WebVTT Node
 Objects</a> that are <i>in the future</i>.</p>
 
 <p algorithm="in the future">A <a>WebVTT Node Object</a> |c| is <dfn abstract-op>in the future</dfn> if, in a