feat: Add project roadmap and initial specifications for dynamic vector props and terrain system

- Created ROADMAP.md outlining core features, implementation phases, and detailed breakdown for the Stickman Sandbox Builder project.
- Introduced plans for dynamic vector props with `PropBlock` specification, including reusable components and utility functions for prop creation.
- Developed a vector terrain system plan detailing the reusable `TerrainBlock` component and associated utility functions for geometry handling.
- Implemented `PhysicsTestHarness` scene for testing physics interactions with dynamic props and terrain.
- Added scripts for `PropBlock`, `PropUtils`, `TerrainBlock`, and `TerrainUtils` to support dynamic prop creation and terrain management.

scenes/physics_test_harness.tscn

@@ -0,0 +1,12 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://scripts/physics_test_harness.gd" id="1_harness"]
+
+[node name="PhysicsTestHarness" type="Node2D"]
+script = ExtResource("1_harness")
+
+[node name="Camera2D" type="Camera2D" parent="."]
+position = Vector2(0, -400)
+zoom = Vector2(0.5, 0.5)
+
+[node name="Environment" type="Node2D" parent="."]