SQL Property Graph Queries (SQL/PGQ)

Here is a prototype implementation of SQL property graph queries
(SQL/PGQ), following SQL:2023.  This was talked about briefly at the
FOSDEM developer meeting, and a few people were interested, so I
wrapped up what I had in progress into a presentable form.

There is some documentation to get started in doc/src/sgml/ddl.sgml
and doc/src/sgml/queries.sgml.

To learn more about this facility, here are some external resources:

* An article about a competing product:
   https://oracle-base.com/articles/23c/sql-property-graphs-and-sql-pgq-23c
   (All the queries in the article work, except the ones using
   vertex_id() and edge_id(), which are non-standard, and the JSON
   examples at the end, which require some of the in-progress JSON
   functionality for PostgreSQL.)

* An academic paper related to another competing product:
   https://www.cidrdb.org/cidr2023/papers/p66-wolde.pdf (The main part
   of this paper discusses advanced functionality that my patch doesn't
   have.)

* A 2019 presentation about graph databases:
   https://www.pgcon.org/2019/schedule/events/1300.en.html (There is
   also a video.)

* (Vik has a recent presentation "Property Graphs: When the Relational
   Model Is Not Enough", but I haven't found the content posted
   online.)

The patch is quite fragile, and treading outside the tested paths will
likely lead to grave misbehavior.  Use with caution.  But I feel that
the general structure is ok, and we just need to fill in the
proverbial few thousand lines of code in the designated areas.