Postgres: Support IS DOCUMENT predicate

[BenSatori]

Adds parsing and round-tripping for PostgreSQL XML IS DOCUMENT and IS NOT DOCUMENT predicates.

PostgreSQL documents these under XML predicates: https://www.postgresql.org/docs/current/functions-xml.html#FUNCTIONS-XML-PREDICATES

The parser handles DOCUMENT contextually after IS, gated by the existing XML expression dialect capability, so it does not become a global keyword.

AI assistance was used in preparing this change.