psql: Split extended query protocol meta-commands in --help=commands

psql: Split extended query protocol meta-commands in --help=commands

Compared to v17 with only \bind able to do extended query protocol work,
v18 has now a total of 11 meta-commands related to the extended query
protocol.  These were all listed under the "General" section of the
--help=commands output and are specialized, bloating the output
generated.

All these meta-commands are moved into a new section called "Extended
Query Protocol", listed at the end of --help=commands.

This split has been suggested by Noah Misch.

Discussion: https://postgr.es/m/20250415213450.1f.nmisch@google.com