This adds a Convert Curves to Legacy Hair operator. It converts all selected curves objects into particle hair systems on their respective surface objects. Existing particle systems with the correct name are updated, otherwise new a new particle system is added.
The purpose of the operator is to make the new curves sculpting tools useful even before all functionality is ported over.

Currently, the generated hair system does not map exactly to the curves object for reasons that I don't fully understand. It's very close though. It might be a drawing issue or the hair strands are resampled somewhere else.

One thing that is still not 100% clear is how to expose this operator in the ui in curves sculpt mode. I created a follow up task for that: T96889.