varlink_generator uses Rust 2015 edition for formatting by default

[erwinschrodinger1]

Description

While using varlink_generator, the function cargo_build_tosource_options appears to default to the Rust 2015 edition for formatting, even when the Cargo.toml specifies a different edition (e.g., 2018 or 2021).

As a result, running cargo fmt --check may produce formatting errors or warnings because rustfmt uses the default 2015 edition instead of respecting the crate's configured edition.

Proposed Solution

Introduce an option to explicitly set the fmt_edition when invoking formatting-related functionality. This will ensure that rustfmt uses the given edition for example via build.rs.

This change would help maintain consistent formatting.

Changes in #125