The i18n-helpers are now available as a stand-alone crate:
https://crates.io/crates/mdbook-i18n-helpers.
Because we cache the Rust binaries in our GitHub workflows, I bumped
the cache prefix to ensure we use a clean cache. Otherwise, Cargo
won’t install the new binaries in mdbook-i18n-helpers because it sees
the old ones from this repository.
* Move Cargo.toml for exercises to exercises directory.
* Create a workspace with both exercises and i18n-helpers.
* Build in CI as well as testing.
* Binaries must have a main function.
* No need for workspaces configuration for caching anymore.
