MSSQL: Support THROW statement

[guan404ming]

Why

The MSSQL THROW statement was missing from the sqlparser, preventing parsing of T-SQL error handling code.

How

• Add THROW keyword and Statement::Throw AST variant with optional error_number, message, state
• Add parse_throw() handling both THROW (re-throw) and THROW num, msg, state forms
• Add tests for literal args, variable args, and argument-less re-throw

[guan404ming]

Hi @iffyio I would like modify the PR title to MSSQL: support THROW statement to better align the convention here but I found there is no edit button here for this pr. Could you help modify it, thanks!

[iffyio]

Done!

[guan404ming]

Thanks! Please let me know if there is anything could be better in this pr, thanks!