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/syncback-tests/UFOWave_matching/expected/Hitbox/Water.model.json5

@@ -1,6 +1,6 @@
 {
   attributes: {
-    Rojo_Ref_SoundGroup: "ServerStorage/__server_muted.model.json5"
+    Rojo_Ref_SoundGroup: "../../__server_muted.model.json5"
   },
   className: "Sound",
   properties: {

rojo-test/syncback-tests/UFOWave_matching/expected/init.meta.json5

@@ -1,6 +1,6 @@
 {
   attributes: {
-    Rojo_Ref_PrimaryPart: "ServerStorage/UFOWave/TsunamiWave",
+    Rojo_Ref_PrimaryPart: "@self/TsunamiWave",
     Speed: 10,
     ZonesPassed: {
       Float32: 0