Model MySQL CREATE TRIGGER statements

[minleejae]

Summary

• replace the unsupported fallback for MySQL CREATE TRIGGER with a dedicated statement AST
• expose definer accounts, trigger timing/event, target table, ordering, and body as structured values
• retain trigger bodies as normal statement nodes so callers can inspect assignments and referenced tables directly

Validation

• ./gradlew spotlessApply test spotlessCheck checkstyleMain checkstyleTest
• executed before/after, insert/update/delete, definer, and ordered-trigger forms against MySQL 8.4