Rewrite tags with copy output rule

[mr-martian]

n: % ;
NP -> n { 1[number=sg]; } ;

Currently [number=sg] is ignored, but it could be handled with SETCLIP.

[mr-martian]

https://github.com/apertium/apertium-recursive/blob/main/src/rtx_compiler.cc#L2006
change needed here-ish