assistant: Add basic current project context (#11828)

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

---------

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

crates/assistant/Cargo.toml

@@ -12,6 +12,7 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 anthropic = { workspace = true, features = ["schemars"] }
+cargo_toml.workspace = true
 chrono.workspace = true
 client.workspace = true
 collections.workspace = true
@@ -41,6 +42,7 @@ smol.workspace = true
 telemetry_events.workspace = true
 theme.workspace = true
 tiktoken-rs.workspace = true
+toml.workspace = true
 ui.workspace = true
 util.workspace = true
 uuid.workspace = true