Make visitor feature no_std-compatible

[LucaCappelletti94]

Enabling the visitor feature without std (e.g. default-features = false, features = ["visitor"] on wasm32-unknown-unknown) currently fails with ~3200 errors. The Visit / VisitMut derive in sqlparser_derive hard-codes ::std::ops::ControlFlow in the generated impls, and src/ast/visitor.rs references String / Vec / Box without importing them from alloc, so neither path resolves once the crate root evaluates #![no_std].

The existing compile-no-std CI job is extended with a --features visitor cargo check on thumbv6m-none-eabi so this regression cannot land silently again.

[iffyio]

LGTM! Thanks @LucaCappelletti94!