fish: handle theme plugins

[dgabka]

Description

Link theme files from installed plugins so that they may be properly loaded with fish_config theme choose <theme>

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}
  

Maintainer CC

No maintainers so reaching out to people with most recent changes:
@natsukium @a-kenji

#3724