4.0.0 for Blender 4.0

Release for Blender 4.0.

Fixed Bugs:

• Fix diffuse colour for textured materials (#12).
• Fix a possible bug with materials containing ShaderNodeGroup nodes.
• Fix an AttributeError when exporting a material with an Image Texture node with no image associated.
• Attempt to export split normals (#10)—I hope this doesn't break more things than it solves.

New Features:

• Specular colour can now be RGB instead of Greyscale.