[Feature] Add setpath for fish shell

[LinasBeres]

Description

This PR adds setpath functionality for the fish shell.

Testing

Local tests on Linux with fish shell.

Checklist

• [x] My code follows the style guidelines of this project
• [x] My changes generate no new warnings
• [x] My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• [x] I have commented my code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I cleaned the commit history and removed any "Merge" commits
• [x] I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license

[njroussel]

Hi @LinasBeres

Thanks for the clear PR, but I don't think we'll be keeping it. AFAIK, every other shell has their own completion format/tool, supporting them all would be a mess, and having only fish is a bit awkward. However, feel free to ship this independently, I'm sure some people will appreciate it.