✨ Support workspace Cargo.toml with package section

[OlivierLDff]

This commit also introduce:

• ✅ Unit test for previous behavior
• 🐛 make description optional field like in other package (inpired from pyproject)

[c4th4r5y5]

Hi @OlivierLDff

Are you aware of any other format distinction for the toml in Rust?

I've dealt with a similar issue when trying to implement the Python preset (#265). It might be worth it to reuse the deep search logic i've put in there to cover more cases that might arise if the format changes.

(For python, I simply do a top-down search of the expected key name and take the first values found. This is somewhat future proof but should require an explicit warning in the readme imo)

[OlivierLDff]

In rust package are either a top key of the cargo.toml are a workspace package, I never saw anything else and I'm not aware if something else exists

[OlivierLDff]

Any news?