fix(mysql): support IF EXISTS/IF NOT EXISTS in trigger/revoke syntax

[h3n4l]

Summary

• add MySQL parser support for CREATE TRIGGER IF NOT EXISTS
• add MySQL parser support for REVOKE IF EXISTS ...
• add MySQL parser support for trailing IGNORE UNKNOWN USER in REVOKE
• add parser regression examples for these statements

Testing

• go test ./mysql -run 'TestMySQLDBSQLParser/examples/(create_trigger_if_not_exists|revoke_if_exists_select|revoke_if_exists_ignore_unknown_user)\.sql$' -count=1
• go test ./mysql -count=1

[Copilot]

Pull request overview

This PR enhances the MySQL parser to support additional conditional and error-handling syntax in DDL statements, specifically adding support for IF EXISTS and IF NOT EXISTS clauses in REVOKE and CREATE TRIGGER statements, as well as the IGNORE UNKNOWN USER clause in REVOKE statements.

Changes:

• Added IF EXISTS support to REVOKE statements
• Added IF NOT EXISTS support to CREATE TRIGGER statements
• Added IGNORE UNKNOWN USER trailing clause support to REVOKE statements

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mysql/MySQLParser.g4	Updated grammar rules to add optional ifNotExists to createTrigger, optional ifExists and ignoreUnknownUser to revoke, and defined new ignoreUnknownUser rule
mysql/mysqlparser_visitor.go	Added VisitIgnoreUnknownUser method to visitor interface (auto-generated)
mysql/mysqlparser_listener.go	Added EnterIgnoreUnknownUser and ExitIgnoreUnknownUser methods to listener interface (auto-generated)
mysql/mysqlparser_base_visitor.go	Implemented VisitIgnoreUnknownUser method in base visitor (auto-generated)
mysql/mysqlparser_base_listener.go	Implemented EnterIgnoreUnknownUser and ExitIgnoreUnknownUser methods in base listener (auto-generated)
mysql/examples/revoke_if_exists_select.sql	Added test example for REVOKE IF EXISTS syntax
mysql/examples/revoke_if_exists_ignore_unknown_user.sql	Added test example for REVOKE IF EXISTS with IGNORE UNKNOWN USER
mysql/examples/create_trigger_if_not_exists.sql	Added test example for CREATE TRIGGER IF NOT EXISTS syntax

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