Implement `next_nearest_neighbors`

Implement next_nearest_neighbors(g, v) which returns the next-to-nearest neighbors of a vertex v in g.

It looks like we could make use of Graphs.neighborhood_dists to create a more general function for outputting the vertices that are the k-nearest neighbors of a specified vertex, for example:

function k_nearest_neighbors(g::AbstractGraph, v, d)
  nds = neighborhood_dists(g, v, d)
  # Filter `nds` for vertices with distances equal to `d`
end

Then next_nearest_neighbors(g, v) = k_nearest_neighbors(g, v, 2).

Originally posted by @mtfishman in https://github.com/mtfishman/ITensorNetworks.jl/pull/18#discussion_r1014849996

Start by locating the graph-neighbor algorithms and reading Graphs.neighborhood_dists, which the issue identifies as the likely basis. Implement next_nearest_neighbors(g, v) so it returns the vertices at distance 2 from v, and verify that behavior against the repository's existing graph tests or test setup.