Extract a BufferStore object from Project (#14037)

This is a ~small~ pure refactor that's a step toward SSH remoting. I've
extracted the Project's buffer state management into a smaller, separate
struct called `BufferStore`, currently in the same crate. I did this as
a separate PR to reduce conflicts between main and `remoting-over-ssh`.

The idea is to make use of this struct (and other smaller structs that
make up `Project`) in a dedicated, simpler `HeadlessProject` type that
we will use in the SSH server to model the remote end of a project. With
this approach, as we develop the headless project, we can avoid adding
more conditional logic to `Project` itself (which is already very
complex), and actually make `Project` a bit smaller by extracting out
helper objects.

Release Notes:

- N/A

crates/project/src/project_tests.rs

@@ -3056,15 +3056,8 @@ async fn test_rescan_and_remote_updates(cx: &mut gpui::TestAppContext) {
         });
     });
 
-    let remote = cx.update(|cx| {
-        Worktree::remote(
-            0,
-            1,
-            metadata,
-            Box::new(CollabRemoteWorktreeClient(project.read(cx).client())),
-            cx,
-        )
-    });
+    let remote =
+        cx.update(|cx| Worktree::remote(0, 1, metadata, project.read(cx).client().into(), cx));
 
     cx.executor().run_until_parked();