Postgres: Support BETWEEN SYMMETRIC / ASYMMETRIC

[BenSatori]

Before this fix, PostgreSQL's BETWEEN SYMMETRIC / BETWEEN ASYMMETRIC modifier wasn't parsed:

SELECT * FROM customers WHERE a NOT BETWEEN SYMMETRIC 10 AND 1;

This adds a symmetric field to Expr::Between and parses the optional SYMMETRIC/ASYMMETRIC keyword after BETWEEN (per the SQL standard / PostgreSQL docs: https://www.postgresql.org/docs/current/functions-comparison.html). ASYMMETRIC is the default behavior and is not round-tripped in the displayed SQL.

AI assistance disclosure: this change was drafted with GitHub Copilot.