fix: add native JSON type support for SQL Server 2025

[DiscountDarcy]

SQL Server 2025 (v17.x) introduces a native JSON data type with binary-optimized storage and full JSON function support.

• typeJson() now returns 'JSON' instead of 'NVARCHAR(MAX)'
• typeJsonb() now returns 'JSON' instead of 'NVARCHAR(MAX)'
• Updated SqlServerSchemaBuilderSpec json/jsonb DDL expectations

[elpete]

Thanks for putting this together. I agree that qb should support SQL Server 2025's native JSON type, but I don't want to change the existing SqlServerGrammar globally because qb still supports SQL Server versions where JSON is not a valid native data type.

I'd like this implemented as a dedicated SqlServer2025Grammar that extends SqlServerGrammar and overrides typeJson() and typeJsonb() to return JSON. The existing grammar should continue returning NVARCHAR(MAX). Please include coverage for both the legacy/default SQL Server grammar and the SQL Server 2025 grammar.

The new grammar should also be documented as an explicit opt-in; we should not select it through ordinary SQL Server auto-discovery unless we add reliable version-aware discovery separately.