Upgrade to Godot 4.3 and add complete Variant type support

- Upgrade godot-cpp submodule to 4.3 branch
- Update GDExtension compatibility_minimum to 4.3
- Add support for PackedVector4Array in variant_to_bert
- Add comprehensive support for all Variant types:
  * Vector types: VECTOR2I, VECTOR3I, VECTOR4I
  * Geometry: RECT2, RECT2I, PLANE, AABB, QUATERNION
  * Transforms: TRANSFORM2D, TRANSFORM3D, BASIS, PROJECTION
  * Special: STRING_NAME, NODE_PATH, RID, CALLABLE, SIGNAL
  * Packed arrays: All packed array types (Byte, Int32, Int64, Float32, Float64, String, Vector2, Vector3, Vector4, Color)
- Update README.md example to show 4.3 compatibility
- Add all necessary variant header includes

Author: fire

README.md

@@ -87,7 +87,7 @@ See [example.gdextension](test/project/example.gdextension) used in the test pro
 [configuration]
 
 entry_symbol = "example_library_init"
-compatibility_minimum = "4.1"
+compatibility_minimum = "4.3"
 
 [libraries]