Enable fish completions even without plugin management support for fish

[mainrs]

Right now RawCommand::Completions relies on config::Shell, which is either Bash or Zsh on the latest stable release. Specifically for that command only, we can rely on clap_complete::Shell as given in their official example. This way one can generate completions for all shells that clap_complete supports even though sheldon's plugin management itself doesn't work with the shells.