BLENDER_USER_RESOURCES accept multiple paths

The idea is to be able to source startup scripts, addons, .. from multiples paths using the new BLENDER_USER_RESOURCES environment variable introduced in 3.4.

As in pipeline, we may have different packages manage by differents teams dev, td, etc...
Also we may also want to use another resources paths for example the Shotgrid toolkit (tk-blender that use a startup script), or another asset manager integration, or external package and be able to add/remove from environment easily for debugging.

Thank you