[css-text-decor-4] Add a link-default for "character"

Bikeshed points it to https://infra.spec.whatwg.org/#code-point
by default.

Author: xfq

css-text-decor-4/Overview.bs

@@ -17,6 +17,10 @@ Link Defaults: css-color-3 (property) color, css-break-3 (dfn) fragment
 Ignored Terms: svg shape, svg shapes, invalid, repeatable list, simple list, valid image
 </pre>
 
+<pre class="link-defaults">
+spec:css-text-3; type:dfn; text:character
+</pre>
+		
 <h2 id=intro>Introduction</h2>
 
 	This is the initial draft of CSS Text Decoration Level 4;