- Detect if we are in a cargo project more reliably.
(e.g. if `rustlings init` is run in the `src/` directory)
- Refuse to initialize rustlings in a non-workspace cargo project.
- Automatically populate the `workspace.members` field if `rustlings init` is
run in a workspace.
This may be considered risky, as there is no guarantee that's what the user
wanted to do. However, it is consistent with the behavior of `cargo new`.
Also, newcomers to Rust are unlikely to accidentally be in a cargo workspace,
as they won't know how to create one in the first place.
The use case for initialization in a workspace is when a workshop organizer
wants to use rustlings alongside other exerices and provide a single
repository with everything in one place.
