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.
