Define function subject collection

Author: aromaa

src/main/java/org/spongepowered/api/service/permission/PermissionService.java

@@ -102,6 +102,12 @@ public interface PermissionService {
      */
     String SUBJECTS_COMMAND_BLOCK = "commandblock";
 
+    /**
+     * The standard identifier for the collection which stores function
+     * subjects.
+     */
+    String SUBJECTS_FUNCTION = "function";
+
     /**
      * The standard identifier for the collection which stores role templates.
      *