Round-trippable show for `Bidiagonal` (#55347)
This changes how a `Bidiagonal` is displayed using the 2-arg `show` to a
form that may be parsed.
After this,
```julia
julia> B = Bidiagonal([1,2,3], [1,2], :U)
3×3 Bidiagonal{Int64, Vector{Int64}}:
1 1 ⋅
⋅ 2 2
⋅ ⋅ 3
julia> show(B)
Bidiagonal([1, 2, 3], [1, 2], :U)
```
The displayed form is a valid constructor now.
