This indirectly also fixes `blend2json.py` because it's build on top of
`blendfile.py`.
Main changes:
* Update .blend file header parsing to support the different header types.
* Support unpacking `LargeBHead8`.
* Use `namedtuple` instead of index access when accessing block header because
the order of items is different now.
* Update comments that were either fully redundant or already outdated.
Pull Request: https://projects.blender.org/blender/blender/pulls/140195
d83bfee347