Expose a method for mutating Parser::index

We have designed an SQL parser using sqlparser's `Parser`, and we wish to implement two helper methods for consuming tokens—similar to `Parser::consume_token` and `Parser::consume_tokens`—but with the ability to accept string inputs and perform case-insensitive matching.

However, since the Parser does not provide a method to mutate the `Parser::index`, we are unable to correctly implement our own `consume_tokens` because we need to revert the parser state if consumption fails mid-way.

We propose adding and exposing a new method in sqlparser's `Parser`, such as `set_index` or `index_mut`, to allow users to mutate the index.

The expected implementation of our `consume_token` and `consume_tokens`:

``` rust
/// Consumes the next token if it matches the expected token, otherwise return false.
///
/// Note, the matching is not case sensitive.
fn consume_token(&mut self, expected: &str) -> bool {
if self.parser.peek_token().to_string().to_uppercase() == *expected.to_uppercase() {
self.parser.next_token();
true
} else {
false
}
}

/// If the current and subsequent tokens exactly match the `tokens` sequence, consume them and returns true.
/// Otherwise, no tokens are consumed and returns false
///
/// Note, the matching is not case sensitive.
fn consume_tokens(&mut self, tokens: &[&str]) -> bool {
let index = self.parser.index();
for token in tokens {
if !self.consume_token(*token) {
// Or: `*self.parser.index_mut() = index;`
self.parser.set_index(index);
return false;
}
}
true
}

```

Commencez par localiser l’implémentation de Rust Parser et ses méthodes existantes index, consume_token et consume_tokens. Examinez ensuite les tests du parser concernant la consommation des tokens, puis exposez un moyen de modifier ou de définir l’index afin qu’une consommation de plusieurs tokens qui échoue puisse restaurer la position précédente.

Rédigé par le modèle d'indexation à partir du texte de l'issue.