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/editor/src/items.rs

@@ -371,7 +371,7 @@ async fn update_editor_from_message(
                     continue;
                 };
                 let buffer_id = BufferId::new(excerpt.buffer_id)?;
-                let Some(buffer) = project.read(cx).buffer_for_id(buffer_id) else {
+                let Some(buffer) = project.read(cx).buffer_for_id(buffer_id, cx) else {
                     continue;
                 };