This is part of a larger refactoring towards a more extensible architecture in Blender (T75724).
- Add new blenloader API that allows decentralizing the .blend file I/O code (rB48075b2c05).
- Use new API in readfile.c and writefile.c.
- Add new callbacks to ModifierTypeInfo (rBb6981d9e48).
- Add new callbacks to IDTypeInfo (rBa4432879082).
- Move remaining id type specific code from readfile.c and writefile.c to IDTypeInfo callbacks.