faust-mode: add copyright to header

loosely follows: https://ben.balter.com/2015/06/03/copyright-notices-for-websites-and-open-source-projects/

Author: magnetophon

faust-mode/header

@@ -28,4 +28,5 @@ declare license "${4:$$
     "MPL-2.0"
     "Unlicense"
   ))}";
-$0
+declare copyright "${5:starting year} - ${6:`(format-time-string "%Y")`$0}, ${7:$3}"
+$0