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/collab/src/tests/integration_tests.rs

@@ -3327,7 +3327,7 @@ async fn test_local_settings(
         let store = cx.global::<SettingsStore>();
         assert_eq!(
             store
-                .local_settings(worktree_b.read(cx).id().to_usize())
+                .local_settings(worktree_b.entity_id().as_u64() as _)
                 .collect::<Vec<_>>(),
             &[
                 (Path::new("").into(), r#"{"tab_size":2}"#.to_string()),
@@ -3346,7 +3346,7 @@ async fn test_local_settings(
         let store = cx.global::<SettingsStore>();
         assert_eq!(
             store
-                .local_settings(worktree_b.read(cx).id().to_usize())
+                .local_settings(worktree_b.entity_id().as_u64() as _)
                 .collect::<Vec<_>>(),
             &[
                 (Path::new("").into(), r#"{}"#.to_string()),
@@ -3375,7 +3375,7 @@ async fn test_local_settings(
         let store = cx.global::<SettingsStore>();
         assert_eq!(
             store
-                .local_settings(worktree_b.read(cx).id().to_usize())
+                .local_settings(worktree_b.entity_id().as_u64() as _)
                 .collect::<Vec<_>>(),
             &[
                 (Path::new("a").into(), r#"{"tab_size":8}"#.to_string()),
@@ -3407,7 +3407,7 @@ async fn test_local_settings(
         let store = cx.global::<SettingsStore>();
         assert_eq!(
             store
-                .local_settings(worktree_b.read(cx).id().to_usize())
+                .local_settings(worktree_b.entity_id().as_u64() as _)
                 .collect::<Vec<_>>(),
             &[(Path::new("a").into(), r#"{"hard_tabs":true}"#.to_string()),]
         )