psql: Add tab completion for pstdin and pstdout in \copy.

psql: Add tab completion for pstdin and pstdout in \copy.

This commit adds tab completion support for the keywords "pstdin" and
"pstdout" in the \copy command. "pstdin" is now suggested after FROM,
and "pstdout" is suggested after TO, alongside filenames and other
keywords.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/20251231183953.95132e171e43abd5e9b78084@sraoss.co.jp