Add pre_visit_ident / post_visit_ident hooks to the Visitor trait

[sabir-akhadov-localstack]

The Visitor / VisitorMut traits had no hook for individual identifiers: Ident derived a no-op Visit, so identifiers could not be observed or rewritten during a traversal. This makes it impossible to, e.g., collect every identifier in a statement or rewrite identifiers in place.

This adds pre_visit_ident / post_visit_ident (default no-op) to both Visitor and VisitorMut, and gives Ident a manual Visit / VisitMut impl that invokes them.

Example — collect identifiers:

struct IdentVisitor { idents: Vec<String> }
impl Visitor for IdentVisitor {
    type Break = ();
    fn pre_visit_ident(&mut self, ident: &Ident) -> ControlFlow<()> {
        self.idents.push(ident.value.clone());
        ControlFlow::Continue(())
    }
}
// "SELECT a, b FROM t" -> ["a", "b", "t"]

Tests added in src/ast/visitor.rs for both the immutable and mutable hooks.

[sabir-akhadov-localstack]

Let me know if I can make this easier to review.

I have a medium sized stack of commits for Snowflake dialect improvements ready to go after this one but I don't want to overwhelm the pipeline.

[sabir-akhadov-localstack]

Hi @iffyio — would you mind kicking off the CI workflow on this PR?
It looks like it's waiting on maintainer approval to run (first-time contributor), so there are no check results yet.
Happy to address any feedback once it's green.

For context, I am working on a local Snowflake emulator in Rust and will be extending the Snowflake dialect extensively.

[iffyio]

LGTM! Thanks @sabir-akhadov-localstack!