Accept IndexStyle in diagind, defaulting to IndexLinear (#52549)
Partly revert and redesign #52115, with `diagind` now accepting an
optional `IndexStyle`, which is `IndexLinear` by default. This should
address the breakages reported in that PR. After this,
```julia
julia> D = Diagonal(1:4)
4×4 Diagonal{Int64, UnitRange{Int64}}:
1 ⋅ ⋅ ⋅
⋅ 2 ⋅ ⋅
⋅ ⋅ 3 ⋅
⋅ ⋅ ⋅ 4
julia> diagind(D)
1:5:16
julia> diagind(D, IndexCartesian())
StepRangeLen(CartesianIndex(1, 1), CartesianIndex(1, 1), 4)
```
---------
Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>
