parse_unnest() does not handle BigQuery WITH OFFSET syntax

[eitsupi]

parse_unnest() does not parse WITH OFFSET [AS alias] for BigQuery dialect. It always sets offset_alias: None.

The other two UNNEST parsing paths in the parser already handle this correctly.

-- Fails to parse with DialectType::BigQuery
SELECT * FROM UNNEST(arr) WITH OFFSET AS pos

The corresponding sqlglot _parse_unnest() handles this case.