change .to to return CSSUnitValue

Author: darrnshn

css-typed-om/Overview.bs

@@ -654,7 +654,7 @@ are represented by subclasses of the {{CSSNumericValue}} interface.
 
         boolean equals(CSSNumberish... value);
 
-        CSSNumericValue to(DOMString unit);
+        CSSUnitValue to(DOMString unit);
         CSSMathSum toSum(DOMString... units);
         // ??? type();