programs.fish: Configurable generation of completions.

[jcollie]

Description

Add a config option to optionally disable the generation of command completions based upon man pages.

Checklist

• [X] Change is backwards compatible.

• [X] Code formatted with ./format.

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

  The tests fail on the master branch for me my changes made no difference.

• [ ] Test cases updated/added. See example.

• [X] 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.

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