pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/28c4b8a05ba5431168e4bc8683f07e03ac911a2c

Modified Files
--------------
src/bin/psql/tab-complete.in.c | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)