[css-fonts-4] Define feature-value-name for font-variant-alternates

#1000

Author: litherum

css-fonts-4/Overview.bs

@@ -3402,6 +3402,8 @@ Media: visual
 Animatable: no
 </pre>
 
+<pre class="prod"><dfn id="feature-value-name-value">&lt;feature-value-name&gt;</dfn> = <<ident>></pre>
+
 For any given character, fonts can provide a variety of alternate
 glyphs in addition to the default glyph for that character.  This
 property provides control over the selection of these alternate