This PR adds synthetic children for std::deque from MSVC's STL. Similar to libstdc++ and libc++, the elements are in a `T**`, so we need to "subscript" twice. The [NatVis for deque](313964b78a/stl/debugger/STL.natvis (L1103-L1112)) uses `_EEN_DS` which contains the block size. We can't access this, but we can access the [constexpr `_Block_size`](313964b78a/stl/inc/deque (L641)). Towards #24834.