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.
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