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.

rojo-test/serve-tests/ref_forward_sync/src/Workspace/MyModel/init.meta.json5

@@ -1,6 +1,6 @@
 {
   "className": "Model",
   "attributes": {
-    "Rojo_Ref_PrimaryPart": "Workspace/MyModel/Target"
+    "Rojo_Ref_PrimaryPart": "@self/Target"
   }
 }