Support MySQL ALTER TABLE rename variants

[minleejae]

Summary

• parse ALTER TABLE old RENAME new and RENAME AS new in addition to RENAME TO new
• expose the exact table-rename keyword as NONE, AS, or TO
• preserve RENAME TO as the default for programmatically constructed AlterExpression instances
• keep column and index rename parsing distinct

Validation

• ./gradlew test spotlessCheck checkstyleMain checkstyleTest
• parser generation remains at the baseline 11 warnings
• all three table-rename forms were executed successfully against MySQL 8.4

PR checklist

• One independently reviewable syntax feature
• Structured AST access; no token reparsing
• Existing rename output remains backward compatible