Similar to `bpy.data.user_map`, it returns a mapping of IDs to all the
filepaths they use.
Fairly basic still, may need to be refined with more options to
control which filepaths are included etc.
Mainly intended to make handling of a production blendfile dependencies
more easy.
Also adds some basic testing of the new feature.
Pull Request: https://projects.blender.org/blender/blender/pulls/127252
2d7068a0d3