Modifiers: New callbacks for reading and writing .blend files

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.

source/blender/modifiers/intern/MOD_meshsequencecache.c

@@ -259,4 +259,6 @@ ModifierTypeInfo modifierType_MeshSequenceCache = {
     /* foreachTexLink */ NULL,
     /* freeRuntimeData */ NULL,
     /* panelRegister */ panelRegister,
+    /* blendWrite */ NULL,
+    /* blendRead */ NULL,
 };