This PR adds the beginnings of current project context to the Assistant.

Currently it supports reading a `Cargo.toml` file and using that to get
some basic information about the project, and its dependencies:

<img width="1264" alt="Screenshot 2024-05-14 at 6 17 03 PM"
src="https://github.com/zed-industries/zed/assets/1486634/cc8ed5ad-0ccb-45da-9c07-c96af84a14e3">

Release Notes:

- N/A

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Co-authored-by: Nate <nate@zed.dev>