Update label to parenthetical long form

To make author line like:
Sönke Bartling, Sascha Friesike (editors)

build/assets/style.csl

@@ -16,6 +16,7 @@
   <macro name="author">
     <names variable="author">
       <name initialize="false" initialize-with="" et-al-min="12" et-al-use-first="10" et-al-use-last="true"/>
+      <label form="long" prefix=" (" suffix=")"/>
       <substitute>
         <names variable="editor"/>
         <text macro="venue"/>