Add Luau-style ref paths migration plan and update related tests

- Introduced a new migration plan for converting `Rojo_Ref_*` attribute paths to Luau require-by-string style, enhancing portability with relative paths.
- Updated existing test snapshots to reflect the new path formats, ensuring consistency across various scenarios.
- Enhanced the `compute_relative_ref_path` and `resolve_ref_path` functions to support the new path semantics, improving path resolution logic.
- Refactored related code in `change_processor`, `syncback`, and `snapshot` modules to accommodate the new reference path handling.

These changes improve the robustness and maintainability of reference path management in the project.

src/serve_session.rs

@@ -176,7 +176,9 @@ impl ServeSession {
         let suppressed_paths = Arc::new(Mutex::new(std::collections::HashMap::new()));
         let ref_path_index = {
             let mut index = crate::RefPathIndex::new();
-            index.populate_from_dir(root_project.folder_location());
+            let tree_guard = tree.lock().unwrap();
+            index.populate_from_dir(root_project.folder_location(), &tree_guard);
+            drop(tree_guard);
             Arc::new(Mutex::new(index))
         };