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-test-snapshots/end_to_end__tests__serve__ref_path_change_target_patch.snap

@@ -15,7 +15,7 @@ body:
             Attributes:
               "!Attributes":
                 Rojo_Ref_PrimaryPart:
-                  "!String": Workspace/MyModel/OtherPart
+                  "!String": "@self/OtherPart"
             PrimaryPart:
               "!Ref": id-4
           id: id-3

rojo-test/serve-test-snapshots/end_to_end__tests__serve__ref_path_multiple_attrs_patch.snap

@@ -15,9 +15,9 @@ body:
             Attributes:
               "!Attributes":
                 Rojo_Ref_CustomRef:
-                  "!String": Workspace/MyModel/OtherPart
+                  "!String": "@self/OtherPart"
                 Rojo_Ref_PrimaryPart:
-                  "!String": Workspace/MyModel/Target
+                  "!String": "@self/Target"
             CustomRef:
               "!Ref": id-4
           id: id-3

rojo-test/serve-test-snapshots/end_to_end__tests__serve__ref_path_new_file_patch.snap

@@ -28,7 +28,7 @@ body:
             Attributes:
               "!Attributes":
                 Rojo_Ref_PrimaryPart:
-                  "!String": Workspace/MyModel/Target
+                  "!String": "@self/Target"
           id: id-3
 packetType: messages
 sessionId: id-1

rojo-test/serve-test-snapshots/end_to_end__tests__serve__ref_path_nonexistent_patch.snap

@@ -15,7 +15,7 @@ body:
             Attributes:
               "!Attributes":
                 Rojo_Ref_PrimaryPart:
-                  "!String": Workspace/NonExistent/Part
+                  "!String": "./NonExistent/Part"
             PrimaryPart: ~
           id: id-3
 packetType: messages