MySQL: Add support for ORDER BY on single-table UPDATE

[tpyo]

Adds parsing support for the MySQL-specific ORDER BY clause in single-table UPDATE statements, as documented in the MySQL 8.4 reference.

Changes:

• Added order_by: Vec<OrderByExpr> field to the Update AST struct
• Added supports_update_order_by() dialect method (default false)
• Added tests for UPDATE ... ORDER BY, UPDATE ... ORDER BY ... LIMIT

Example syntax that would be supported:

UPDATE foo SET bar = false WHERE foo = true ORDER BY foo ASC;