Keyword Support for IGNORE NULLS

[xterao]

Description
Evaluate whether IGNORE NULLS, used in BigQuery and MySQL, should be included as a keyword token.

• In PostgreSQL, IGNORE NULLS is not implemented. The default behavior is equivalent to RESPECT NULLS.
• Reference: [PostgreSQL Documentation – Window Functions](https://www.postgresql.org/docs/current/functions-window.html)

Expected Enhancement
When implementing database switching options, add IGNORE NULLS as a keyword token for databases that support it.

Notes

• Ensure compatibility across supported dialects.
• Prevent IGNORE NULLS from being incorrectly applied in PostgreSQL where it is not valid.