Move from mamba to pixi

[Hofer-Julian]

After #589, #381 and countless TeamCity troubles, it is time to try something new

[Hofer-Julian]

This PR is ready to review from my side.
Apart from having a lock file and a sane way of updating environments, it also significantly simplifies our contributor docs.
I tested it already on TeamCity with a job I couldn't get to work with conda and it works fine. It even caches between runs: https://dpcbuild.deltares.nl/buildConfiguration/Ribasim_Ribasim_TestRibasimCliWindows?branch=pixi&mode=builds

[visr]

For consistency this perhaps should also depend on quartodoc-build?

[visr]

This looks great! Install worked flawlessly for me.

So the (huge!) lock file basically pins all our non-julia dependencies, right? Will we manually update every now and then?

Looking at the pixi.toml and thinking about the things I often run together, I think some more combined tasks would be nice to add.

pixi run docs
  build-julia-docs
  quarto-preview
pixi run tests
  lint
  test-ribasim-python
  test-ribasim-core  # TODO
pixi run build
  build-libribasim  # TODO
  build-ribasim-cli  # TODO
pixi run codegen
  generate-schema  # TODO
  generate-python  # TODO
  lint
pixi run generate-testmodels  # TODO add support for args, e.g. pixi run generate-testmodels basic, useful for debugging model writing, where you don't want a flood of errors
pixi run ribasim  # TODO something like using Ribasim; Ribasim.run(ARGS)
pixi run qgis  # I thing qgis is in conda-forge these days, or is that too big a dependency?
  # should first copy the qgis folder to the plugins
pixi run update-qgis-plugin  # TODO copy the qgis folder to the plugins
pixi run resolve-manifests  # run pkg resolve on the checked in manifests

And lastly maybe some combination of these that amounts to doing pretty much all the checks that CI does, but locally. So building docs, running tests.