psql: Improve tab completion for COPY ... STDIN/STDOUT.

psql: Improve tab completion for COPY ... STDIN/STDOUT.

This commit enhances tab completion for both COPY FROM and COPY TO
commands to suggest STDIN and STDOUT, respectively.

To make suggesting both file names and keywords easier, it introduces
a new COMPLETE_WITH_FILES_PLUS() macro.

Author: Yugo Nagata <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/20250605100835.b396f9d656df1018f65a4556@sraoss.co.jp