This is part of a greater blenloader decentralization effort (T76372). For modifiers the goal is that fewer files have to be modified when a new modifier is added. This patch just adds the `blendWrite` and `blendRead` callbacks to `ModifierTypeInfo` but does not change any other code yet. In the next steps, modifier specific code will be moved from `writefile.c` and `readfile.c` into their corresponding `MOD_*` files.