pgsql: Add psql PROMPT variable for search_path.

Add psql PROMPT variable for search_path.

The new %S substitution shows the current value of search_path.
Note that this only works when connected to Postgres v18 or newer,
since search_path was first marked as GUC_REPORT in commit
28a1121fd9. On older versions that don't report search_path, %S is
replaced with a question mark.

Suggested-by: Lauri Siltanen <lauri(dot)siltanen(at)gmail(dot)com>
Author: Florents Tselai <florents(dot)tselai(at)gmail(dot)com>
Reviewed-by: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Reviewed-by: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CANsM767JhTKCRagTaq5Lz52fVwLPVkhSpyD1C%2BOrridGv0SO0A%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3ce55f413cdf70b1bc4724052fb4eacf9de239a

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 11 +++++++++++
src/bin/psql/prompt.c | 11 +++++++++++
2 files changed, 22 insertions(+)