MSSQL: Add support for TRAN shorthand

[guan404ming]

Why

MSSQL supports TRAN as a shorthand for TRANSACTION in BEGIN/COMMIT/ROLLBACK statements, but the
parser did not recognize it.

How

• Add TRAN variant to BeginTransactionKind enum and the keywords list
• Accept TRAN alongside TRANSACTION and WORK in BEGIN, COMMIT, and ROLLBACK parsing
• Add tests for BEGIN TRAN, COMMIT TRAN, and ROLLBACK TRAN