VTKExporter

As suggested by @fredrikekre on Slack during the discussions about #692, this issue contains the notes for one potential way this could be implemented on top of #692

Having a type that can cache the transformation to the vtk data structures makes sense to avoid having to redo this each time step when using a `VTKFileCollection`.

Furthermore, this could then be customized to contain the special mappings required for e.g. discontinuous interpolations ([xref](https://github.com/Ferrite-FEM/Ferrite.jl/pull/692#issuecomment-1760034573)) and user customizations (perhaps for, e.g., [IGA/Embedded](https://github.com/Ferrite-FEM/Ferrite.jl/pull/692#issuecomment-1752423362), @lijas?).

For example, we could have
```julia
struct VTKFile{VTK<:WriteVTK.VTKFile, VTKExp<:VTKExporter}
vtk::VTK
vtk_exporter::VTKExp
end
```