`SHOW FUNCTIONS` allowed syntax does not match source/intent

SHOW FUNCTIONS
https://github.com/sqlparser-rs/sqlparser-rs/blob/7282ce22f9f031a2c9fd5831427f01f2f0dbb978/src/ast/mod.rs#L2450-L2453

it allow syntax
```
SHOW FUNCTIONS
SHOW FUNCTIONS LIKE 'pattern'
SHOW FUNCTIONS ILIKE 'pattern'
SHOW FUNCTIONS WHERE
```

however, it's also **documented** to match Presto's SHOW FUNCTIONS
which doesn't allow ILIKE nor WHERE (and wouldn't know how the WHERE should be applied)
https://prestodb.io/docs/current/sql/show-functions.html

worth noting that Trino's SHOW FUNCTIONS is similar, but allows additional schema filter
https://trino.io/docs/current/sql/show-functions.html

從 src/ast/mod.rs 的第 2450-2453 行開始，將文件中記載的 SHOW FUNCTIONS 文法與連結的 Presto 和 Trino 參考進行比較。確定此 parser 預計支援哪種方言行為，然後調整接受的語法及其文件，使支援的形式保持一致。

由索引模型根據 Issue 內容生成。