Add supplement for addendum

dhsn.typ

@@ -34,7 +34,7 @@
   )
 
 ]
-#let addendum(body) = {
+#let addendum(supplement: "Anhang", body) = {
   pagebreak()
   counter(heading).update(0)
   set heading(numbering: none)
@@ -52,7 +52,7 @@
   set figure(kind: raw, supplement: "Anhang")
   counter(figure.where(kind: raw)).update(0)
 
-  [= Anhang <dhsn.addendum>]
+  [= #supplement <dhsn.addendum>]
   body
   pagebreak()
 }