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
}

```

Start by locating the Rust Parser implementation and its existing index, consume_token, and consume_tokens methods. Check the parser tests around token consumption, then expose a way to mutate or set the index so failed multi-token consumption can restore the prior position.

Written by the indexing model from the issue text.