feat: make DatasetVersion repr valid Julia code

[mortenpi]

This makes sure that the short DatasetVersion string representation is a valid Julia code:

julia> dataset.versions
2-element Vector{JuliaHub.DatasetVersion}:
 JuliaHub.dataset(("username", "example-dataset")).versions[1]
 JuliaHub.dataset(("username", "example-dataset")).versions[2]

Also fix a doctest in the DatasetVersion docstring, which now in turn exercises the show methods for DatasetVersion. Also, add a fix-doctests Make command.