Move NavigationMeshSourceGeometryData(2D/3D) to the 2D/3D subfolders

[aaronfranke]

Follow-up to other organization PRs like PR #83133. This PR does the following:

• Move NavigationMeshSourceGeometryData2D into scene/resources/2d
• Move NavigationMeshSourceGeometryData3D into scene/resources/3d
• Move NavigationPolygon2D into scene/resources/2d
  • Note: Can't move the 3D equivalent NavigationMesh3D because it's needed for 2D navigation.
• In NavigationServer3D, disable parse_source_geometry_data, bake_from_source_geometry_data, bake_from_source_geometry_data_async, and is_baking_navigation_mesh when compiling without 3D.
  • NavigationServer3D is still used when compiling without 3D, but these methods are 3D-only. NavigationServer2D has separate methods with the same names that operate on the 2D versions of these classes.

[akien-mga]

Thanks!