Refactor: Make it possible to share a remote worktree (#12775)

This PR is an internal refactor in preparation for remote editing. It
restructures the public interface of `Worktree`, reducing the number of
call sites that assume that a worktree is local or remote.

* The Project no longer calls `worktree.as_local_mut().unwrap()` in code
paths related to basic file operations
* Fewer code paths in the app rely on the worktree's `LocalSnapshot`
* Worktree-related RPC message handling is more fully encapsulated by
the `Worktree` type.

to do:
* [x] file manipulation operations
* [x] sending worktree updates when sharing

for later
* opening buffers
* updating open buffers upon worktree changes

Release Notes:

- N/A

crates/collab/src/tests/editor_tests.rs

@@ -83,10 +83,7 @@ async fn test_host_disconnect(
     let project_b = client_b.build_dev_server_project(project_id, cx_b).await;
     cx_a.background_executor.run_until_parked();
 
-    assert!(worktree_a.read_with(cx_a, |tree, _| tree
-        .as_local()
-        .unwrap()
-        .has_update_observer()));
+    assert!(worktree_a.read_with(cx_a, |tree, _| tree.has_update_observer()));
 
     let workspace_b = cx_b
         .add_window(|cx| Workspace::new(None, project_b.clone(), client_b.app_state.clone(), cx));
@@ -123,10 +120,7 @@ async fn test_host_disconnect(
 
     project_b.read_with(cx_b, |project, _| project.is_read_only());
 
-    assert!(worktree_a.read_with(cx_a, |tree, _| !tree
-        .as_local()
-        .unwrap()
-        .has_update_observer()));
+    assert!(worktree_a.read_with(cx_a, |tree, _| !tree.has_update_observer()));
 
     // Ensure client B's edited state is reset and that the whole window is blurred.