Added support for Blender 5.2

[LutBox]

Verified against Blender 5.2.0 LTS (Python 3.13): the full test suite, an install as extension and as legacy add-on, and a W3D/W3X export and re-import roundtrip.

Replace all deprecated API usage, so the add-on no longer emits a single deprecation warning on 5.2:

• Material.blend_method -> Material.surface_render_method. The old property has no effect in EEVEE Next, so transparency of imported materials was silently lost since Blender 4.2.
• Material.show_transparent_back -> Material.use_transparency_overlap
• Material.use_nodes, which gets removed in Blender 6.0. The node tree is only requested when the material does not have one yet.
• Mesh.vertex_colors -> Mesh.color_attributes
• MeshUVLoopLayer.data -> MeshUVLoopLayer.uv

All of them go through version guarded helpers, the add-on keeps working on the older Blender versions the CI covers.

Make the add-on installable as an extension:

• add blender_manifest.toml, the released archive now works both as an extension (Blender 4.2+) and as a legacy add-on archive.
• use relative imports inside the package. As an extension the package is named bl_ext..io_mesh_w3d, so the absolute io_mesh_w3d.* imports failed to resolve and enabling the add-on errored out.
• skip the bundled updater when running as an extension, extensions are updated through their repository instead.

Bugfixes found along the way:

• import crashed on Blender 4.1: Mesh.use_auto_smooth was removed in 4.1, not in 4.2 as the version guard assumed.
• w3x writing relied on the truth value of an xml element, which is deprecated in Python and will always be True in a future version. Reading it as a child count keeps the intended meaning.

[Tarcontar]

Looks good to me, great job, thanks! :)

[Tarcontar]

@LutBox please rebase on current master branch so the CI can run

[Tarcontar]

@LutBox sorry i made a mistake with the URL, now it is fixed on master branch and CI finally works again, could you please rebase once more?