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/terminal_view/src/terminal_view.rs

@@ -1301,13 +1301,7 @@ mod tests {
             .unwrap();
 
         let entry = cx
-            .update(|cx| {
-                wt.update(cx, |wt, cx| {
-                    wt.as_local()
-                        .unwrap()
-                        .create_entry(Path::new(""), is_dir, cx)
-                })
-            })
+            .update(|cx| wt.update(cx, |wt, cx| wt.create_entry(Path::new(""), is_dir, cx)))
             .await
             .unwrap()
             .to_included()