Add support for Postgres 18

[lfittl]

(against 17-latest for the review, but for release we will instead rename the branch to 18-latest)

TODO

• [ ] Fix "#include <xlocale.h>" error with CI (and presumably any Linux)
• [ ] Fix deparser differences/crashes (test/deparse)
• [ ] Fix PL/pgSQL parsing crash (test/parse_plpgsql)