pgsql: Provide a test for variable existence in psql

Provide a test for variable existence in psql

"\if :{?variable_name}" will be translated to "\if TRUE" if the variable
exists and "\if FALSE" otherwise. Thus it will be possible to execute code
conditionally on the existence of the variable, regardless of its value.

Fabien Coelho, with some review by Robins Tharakan and some light text
editing by me.

Discussion: https://postgr.es/m/alpine.DEB.2.20.1708260835520.3627@lancre

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d57c7a7c506276597af619bdb8c62fa5b592745a

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 10 +++++++++
src/bin/psql/psqlscanslash.l | 18 ++++++++++++++++
src/fe_utils/psqlscan.l | 42 ++++++++++++++++++++++++++++++++++++-
src/include/fe_utils/psqlscan_int.h | 2 ++
src/test/regress/expected/psql.out | 26 +++++++++++++++++++++++
src/test/regress/sql/psql.sql | 18 ++++++++++++++++
6 files changed, 115 insertions(+), 1 deletion(-)