Add Cargo.lock to .gitignore

[aymannel]

This PR adds Cargo.lock to the .gitignore file.

For libraries and shared crates, Cargo.lock should not be committed. Cargo will automatically resolve and generate a lockfile for consumers of the library, ensuring they can update dependencies according to their own environments and requirements.

Keeping it out of version control avoids unnecessary merge conflicts and noisy diffs caused by dependency updates that are not directly relevant to the library code.

[Ectras]

It used to be added to the .gitignore for libraries by default, but this is no longer the case. In essence, it's a trade-off (see here):

Advantage: Determinism / Reproducibility, especially for CI
Disadvantage: Tricky merge conflicts. When I encounter those, I usually merge the Cargo.toml and regenerate the Cargo.lock, but this can update other dependencies as well