Zellij: add fish completions & autostart options

[tennox]

Description

First, I just wanted to add completions to the fish integration, then I figured I often want completions but not autostart, then I added the autostart options listed in zellij docs, and finally adapted the tests.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@mainrs