[cssom-1] Add CSSImportRule.supportsText attribute

Fixes w3c#8710

Author: CanadaHonk

cssom-1/Overview.bs

@@ -2046,6 +2046,7 @@ interface CSSImportRule : CSSRule {
   [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
   [SameObject] readonly attribute CSSStyleSheet styleSheet;
   readonly attribute CSSOMString? layerName;
+  readonly attribute CSSOMString? supportsText;
 };
 </pre>
 
@@ -2064,6 +2065,9 @@ The <dfn attribute for=CSSImportRule>layerName</dfn> attribute must return the [
 or an empty string if the layer is anonymous,
 or null if the at-rule does not declare a layer.
 
+The <dfn attribute for=CSSImportRule>supportsText</dfn> attribute must return the <<supports-condition>> declared in the at-rule itself,
+or null if the at-rule does not declare a supports condition.
+
 Note: An <code>@import</code> at-rule always has an associated <a>CSS style sheet</a>.