Impl core::error::Error instead of std::error::Error

[DeppLearning]

Hey there,

thanks for the great library! I noticed that the Error trait is not implemented for BuilderErrors when the std feature is inactive. Rust 1.81 stabilized core::error::Error, it would be nice if this crate could make use of it and lift the former restriction. Since, as implemented, this would be a breaking change, I'm wondering what the preferred way forward is. I can think of

• add a feature flag, like core_error that opts into using core::error::Error instead of the std version
• use something like https://crates.io/crates/rustversion/ to gate the core version behind a #[rustversion::since(1.81)]
• others?