Specialize copy for Diagonal (#52500)
On master
```julia
julia> copy(Diagonal(1:4)) |> typeof
Diagonal{Int64, Vector{Int64}}
```
This PR
```julia
julia> copy(Diagonal(1:4)) |> typeof
Diagonal{Int64, UnitRange{Int64}}
```
Similar methods already exist for `Bidiagonal` and `Tridiagonal`, but
this was missing for `Diagonal`.
