PostgreSQL: Support GROUP BY ALL and DISTINCT

[BenSatori]

PostgreSQL accepts ALL or DISTINCT before a GROUP BY expression list, but the parser currently treats ALL as the complete clause. For example, SELECT a FROM t GROUP BY ALL a fails to parse.

This adds a dialect-gated group-by modifier AST representation and supports both documented forms while preserving the existing bare GROUP BY ALL syntax in other dialects.

PostgreSQL documentation: https://www.postgresql.org/docs/current/sql-select.html#SQL-GROUPBY

This contribution was created with AI assistance.