Broadcasting `uconvert` over a range could return a range

When broadcasting a unit (as a function) over a range, it returns a range. But when using `uconvert`, it allocates a vector instead:

```julia
julia> x = (1:2:5) * u"m"
(1:2:5) m

julia> x .|> u"cm"
(100:200:500) cm

julia> uconvert.(u"cm", x)
3-element Vector{Quantity{Int64, 𝐋, Unitful.FreeUnits{(cm,), 𝐋, nothing}}}:
100 cm
300 cm
500 cm
```

Use the reproducer in the issue as the first check, comparing function broadcasting over the range with `uconvert.(u"cm", x)`. Trace the relevant Julia broadcasting behavior and Unitful implementation. Done means conversion preserves the range form, with coverage for the shown example.

Written by the indexing model from the issue text.