Move Ribasim XMI Wrapper into imod coupler

[deltamarnix]

Ribasim XMI Wrapper is now a separate package in the Ribasim repository. This is not required, as it's so small and won't grow in the future. I propose that we move the class into the imod coupler repo next to the other kernel wrappers.

Then in the Ribasim repo, we can make a PR that removes CI/CD and the ribasim_api folder.
It might be that we also need to remove a conda-forge recipe.

The alternative for users of this class is that they simply copy-paste it when they want to work with it separately without the imod coupler. That is always an option of course.

@visr Am I missing anything that could be removed?

[visr]

Yes that is most of it. There are some mentions in the docs, and pixi.toml. It might be nice to change the docs with a link to the code in case they want to copy-paste.

And we should see how to properly deprecate the package on PyPI and conda-forge.