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/src/ambient_context/recent_buffers.rs

@@ -1,6 +1,8 @@
 use gpui::{Subscription, Task, WeakModel};
 use language::Buffer;
 
+use crate::{LanguageModelRequestMessage, Role};
+
 pub struct RecentBuffersContext {
     pub enabled: bool,
     pub buffers: Vec<RecentBuffer>,
@@ -23,3 +25,13 @@ impl Default for RecentBuffersContext {
         }
     }
 }
+
+impl RecentBuffersContext {
+    /// Returns the [`RecentBuffersContext`] as a message to the language model.
+    pub fn to_message(&self) -> Option<LanguageModelRequestMessage> {
+        self.enabled.then(|| LanguageModelRequestMessage {
+            role: Role::System,
+            content: self.message.clone(),
+        })
+    }
+}