Cargo-backed Rust binary targets are now supported

Adds a new cargo_binary target type to the built-in Rust frontend. Cargo-backed Rust binaries can now be declared in fabrik.star files, and fabrik run will build them with cargo build --locked --package <pkg> --bin <bin>. This makes it easier to adopt Fabrik in existing Rust workspaces without first converting every crate to fine-grained targets.