Process library tracking issue

After merging #1146, there is now a minimal low-level process API. There are still things to do:

- [ ] The API is relatively low-level and un-Effekt-y. We should build a nice interface on top.
- In particular, the current API uses lots of boxed callbacks that always need indirections via global state or similar.
- [ ] `spawn`ing a process currently uses a mutable builder for the options it gets passed (see https://github.com/effekt-lang/effekt/pull/1146#discussion_r2514477040 ). We could come up with a better interface here.
- [ ] Some cases (e.g. https://github.com/effekt-lang/effekt/pull/1146#discussion_r2514480432 ) still need handling for certain kinds of errors.
- [ ] Handle signals (https://github.com/effekt-lang/effekt/pull/1146#discussion_r2514507658 )