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 <[email protected]>

Author: jishnub

src/diagonal.jl

@@ -150,6 +150,8 @@ end
     r
 end
 
+axes(D::Diagonal) = (ax = axes(D.diag, 1); (ax, ax))
+
 @inline function Base.isstored(D::Diagonal, i::Int, j::Int)
     @boundscheck checkbounds(D, i, j)
     if i == j

test/diagonal.jl

@@ -460,6 +460,12 @@ Random.seed!(1)
     end
 end
 
+@testset "axes" begin
+    v = OffsetArray(1:3)
+    D = Diagonal(v)
+    @test axes(D) isa NTuple{2,typeof(axes(v,1))}
+end
+
 @testset "rdiv! (#40887)" begin
     @test rdiv!(Matrix(Diagonal([2.0, 3.0])), Diagonal(2:3)) == Diagonal([1.0, 1.0])
     @test rdiv!(fill(3.0, 3, 3), 3.0I(3)) == ones(3,3)