Flattens a nested "block matrix" into a plain matrix.
Berbeda dari referensi .wl: `ArrayFlatten[a, r]`
ArrayFlatten[{{m11, m12, …}, {m21, m22, …}, …}]
ArrayFlatten[a, r]
ArrayFlatten[{{{{1, 2}, {3, 4}}, {{5, 6}, {7, 8}}}}]
→ {{1, 2, 5, 6}, {3, 4, 7, 8}}