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/settings/src/settings_store.rs

@@ -632,8 +632,6 @@ impl SettingsStore {
             }
 
             // If the global settings file changed, reload the global value for the field.
-            project_settings_stack.clear();
-            paths_stack.clear();
             if changed_local_path.is_none() {
                 if let Some(value) = setting_value
                     .load_setting(
@@ -653,6 +651,8 @@ impl SettingsStore {
             }
 
             // Reload the local values for the setting.
+            paths_stack.clear();
+            project_settings_stack.clear();
             for ((root_id, path), local_settings) in &self.raw_local_settings {
                 // Build a stack of all of the local values for that setting.
                 while let Some(prev_entry) = paths_stack.last() {