feat: support MySQL named foreign key and check drops

[minleejae]

In MYSQL mode, support ALTER TABLE t DROP FOREIGN KEY fk1 and DROP CHECK c1, including quoted names and combined ALTER actions. Expose the deletion target through getConstraintName() and the action through DROP_FOREIGN_KEY / new DROP_CHECK, so callers can inspect, rename or construct the action without editing raw SQL.

Share special DROP rendering between parser-created actions and directly constructed AlterExpression nodes. Preserve the existing parenthesized foreign-key form and other DROP branches. Includes usage documentation, AST mutation/construction tests and parse/deparse/reparse checks.

Validation: full Gradle check on Java 17; six valid DDL fixtures executed in original and rendered form on MySQL 8.4, with catalog checks confirming the target constraints were removed. New named alternatives are selected only by Dialect.MYSQL.

Syntax reference: https://dev.mysql.com/doc/refman/8.4/en/alter-table.html