Add support for band

[dlfivefifty]

The following that works for a BandedMatrix and should also work with a ToeplitzMatrix:

A[band(1)] # get the first super-diagonal as a vector
A[band(-1)] = 2  # set all entries on the first sub-diagonal to 2

This is accomplished by band returning a type Band. Maybe Band could be moved into another package that both ToeplitzMatrices.jl and BandedMatrices.jl depend on? (MatrixBands.jl?)

[MikaelSlevinsky]

could have antiband for Hankel matrices 😆

[jishnub]

With the addition of package extensions in Julia v1.9, it seems like a good time to revive this