`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

Comece nas linhas 2450-2453 de src/ast/mod.rs e compare a gramática documentada de SHOW FUNCTIONS com as referências vinculadas do Presto e do Trino. Determine qual comportamento de dialeto este parser pretende oferecer suporte e, em seguida, alinhe a sintaxe aceita e sua documentação para que as formas suportadas sejam consistentes.

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