bug[next]: Fix implicit offset provider

[tehrengruber]

#1484 introduced support for writing field(I+1). The implicit offset providers we generate for this accidentally also included local dimensions, which broke e.g. Icon4Py with errors like:

                    if not dim.kind == common.DimensionKind.VERTICAL:
>                       raise ValueError(
                            "Mapping an offset to a horizontal dimension in unstructured is not allowed."
                        )
E                       ValueError: Mapping an offset to a horizontal dimension in unstructured is not allowed.

This PR fixes that by only including horizontal and vertical, but not local dimensions in the implicit offset providers.

[SF-N]

Thanks, this looks good to me.