[BUG] exists, sample and extend cannot be used as unquoted identifiers #2772

JSqlParser version: master 8d12a5d7a (5.5-SNAPSHOT); extend has failed since 5.2, exists and sample have failed since these tokens were reserved. RDBMS: all.

The following statements fail on master although all three words are non-reserved in the common dialects:

SELECT tb_name, exists FROM entity ORDER BY exists;
SELECT * FROM sample;
UPDATE t SET extend = 'x' WHERE id = 1;

They reach JSqlParser through downstream SQL rewriting tools (baomidou/mybatis-plus#7029, #5956, #6504, #6871), where users cannot rename their columns or tables.

Root cause: K_EXISTS, K_SAMPLE and K_EXTEND are reserved tokens for specific operator positions (the EXISTS predicate and JSON_TABLE flag, Vertica table sampling, ksqDB pipe operators) but were never added to the reserved-keyword-as-identifier alternatives of RelObjectName(), unlike earlier dual-use keywords such as EXTENDED (#1525) or GROUPS (#2473).

A PR follows.