Centralize project context provided to the assistant (#11471)

This PR restructures the way that tools and attachments add information
about the current project to a conversation with the assistant. Rather
than each tool call or attachment generating a new tool or system
message containing information about the project, they can all
collectively mutate a new type called a `ProjectContext`, which stores
all of the project data that should be sent to the assistant. That data
is then formatted in a single place, and passed to the assistant in one
system message.

This prevents multiple tools/attachments from including redundant
context.

Release Notes:

- N/A

---------

Co-authored-by: Kyle <kylek@zed.dev>

crates/semantic_index/src/semantic_index.rs

@@ -163,6 +163,10 @@ impl ProjectIndex {
         self.project.clone()
     }
 
+    pub fn fs(&self) -> Arc<dyn Fs> {
+        self.fs.clone()
+    }
+
     fn handle_project_event(
         &mut self,
         _: Model<Project>,