Built-in target types exposed as a Starlark prelude

Built-in Rust target types are no longer hardcoded in Rust. They are now defined in a bundled prelude/rust.star file that wraps the low-level target(kind, name, srcs, deps) primitive. This means the built-in types use the same extension contract that third-party plugins will use, and you can also call target(...) directly for custom kinds.