This is part of a larger refactoring towards a more extensible architecture in Blender (T75724).
[X] Add new blenloader API that allows decentralizing the .blend file I/O code (rB48075b2c05).
[X] Use new API in `readfile.c` and `writefile.c`.
[X] Add new callbacks to `ModifierTypeInfo` (rBb6981d9e48).
[X] Add new callbacks to `IDTypeInfo` (rBa4432879082).
[ ] Move remaining id type specific code from `readfile.c` and `writefile.c` to `IDTypeInfo` callbacks.
[ ] Add new callbacks for nodes and sockets.
[ ] Add new callbacks for grease pencil modifiers.
[ ] Add new callbacks for space types.