Specialize indexing a CartesianIndices with a CartesianIndex StepRangeLen

[jishnub]

Since a CartesianIndices may be seen as an nD range, indexing it with a range in a specific direction would return a range. This is the nD equivalent of indexing a 1D range with another range, which returns a range as the result.
For example,

julia> C = CartesianIndices((3, 3))
CartesianIndices((3, 3))

julia> collect(C)
3×3 Matrix{CartesianIndex{2}}:
 CartesianIndex(1, 1)  CartesianIndex(1, 2)  CartesianIndex(1, 3)
 CartesianIndex(2, 1)  CartesianIndex(2, 2)  CartesianIndex(2, 3)
 CartesianIndex(3, 1)  CartesianIndex(3, 2)  CartesianIndex(3, 3)

julia> using LinearAlgebra

julia> r = diagind(C, IndexCartesian())
StepRangeLen(CartesianIndex(1, 1), CartesianIndex(1, 1), 3)

julia> C[r] # returns a materialized range
3-element Vector{CartesianIndex{2}}:
 CartesianIndex(1, 1)
 CartesianIndex(2, 2)
 CartesianIndex(3, 3)

After this PR,

julia> C[r] # returns the range without materializing it
StepRangeLen(CartesianIndex(1, 1), CartesianIndex(1, 1), 3)

julia> C[r] |> collect
3-element Vector{CartesianIndex{2}}:
 CartesianIndex(1, 1)
 CartesianIndex(2, 2)
 CartesianIndex(3, 3)

Such operations might find utility in sparse matrix constructions, when one needs a range of indices to populate. See e.g., JuliaSparse/SparseArrays.jl#600