Should `LinearAlgebra.fzero` be declared public?

https://github.com/JuliaLang/julia/blob/5e4669c7403c301985f35d2c8754b184cd73ab05/stdlib/LinearAlgebra/src/structuredbroadcast.jl#L142-L151

This method is useful in general, as it allows custom types to hook into the structure-preserving broadcast mechanism without having to define their own methods for each function. As an example:
```julia
julia> using FillArrays, LinearAlgebra

julia> Diagonal(1:4) .+ Zeros(4,4)
4×4 Matrix{Float64}:
1.0 0.0 0.0 0.0
0.0 2.0 0.0 0.0
0.0 0.0 3.0 0.0
0.0 0.0 0.0 4.0

julia> LinearAlgebra.fzero(x::Zeros) = zero(eltype(x))

julia> Diagonal(1:4) .+ Zeros(4,4)
4×4 Diagonal{Float64, Vector{Float64}}:
1.0 ⋅ ⋅ ⋅
⋅ 2.0 ⋅ ⋅
⋅ ⋅ 3.0 ⋅
⋅ ⋅ ⋅ 4.0
```

Perhaps this could also be done through an `isstructuredmatrix` trait that is made public.

cc: @dlfivefifty @dkarrasch

Start with stdlib/LinearAlgebra/src/structuredbroadcast.jl, especially the fzero method at lines 142–151, and review how custom types hook into structure-preserving broadcast. Compare making LinearAlgebra.fzero public with exposing an isstructuredmatrix trait, using the FillArrays example as a behavior check. Done means the public extension point is chosen and documented or tested consistently.

Written by the indexing model from the issue text.