[libcxx][iterator] adds `std::ranges::next`

Implements part of P0896 'The One Ranges Proposal'.
Implements [range.iter.op.next].

Depends on D101922.

Differential Revision: https://reviews.llvm.org/D102563

NOKEYCHECK=True
GitOrigin-RevId: 857fa7b7b1872a4a9b4072b6e83c85524c7229af
10 files changed