Improve initialization in workspace

- 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.