Forward `axes` to the parent for a `Diagonal` (#50514)
Given that [quite a
few](https://juliahub.com/ui/Search?q=axes%5Ba-zA-Z%5C%28%5C%29%3A%5D%2BDiagonal&type=code&r=true)
packages define `axes(d::Diagonal{T, CustomVector{T}})` as `ax =
parent(d); (ax,ax)`, perhaps it makes sense to have this defined in
`LinearAlgebra`.
After this,
```julia
julia> using StaticArrays, StructArrays, LinearAlgebra
julia> D = Diagonal(StructArray{Complex{Int}}((SA[1,2], SA[1,2])))
2×2 Diagonal{Complex{Int64}, StructVector{Complex{Int64}, @NamedTuple{re::SVector{2, Int64}, im::SVector{2, Int64}}, Int64}} with indices SOneTo(2)×SOneTo(2):
1+1im ⋅
⋅ 2+2im
julia> axes(D)
(SOneTo(2), SOneTo(2))
```
The static sizes are preserved.
---------
Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>
