Add basic completion for fish

[zvyn]

This is a subset of #151, only implementing the most relevant completion (see #118), i.e. completing password names.

[rani-i]

On macos you should source
/usr/local/share/fish/vendor_completions.d/pass.fish
and not only
source "/usr/share/fish/vendor_completions.d/pass.fish"

therefore, another approach is required

[mserajnik]

I have expanded a bit upon this and added completions for all of pass-otp's subcommands as well. I have also implemented a rather hacky way to figure out the path to the original fish completions semi-dynamically.

Imo, it's not fit for a PR and should ultimately be solved in a better way, but I have put the completions into a repository, if anyone else wants to use them. You can simply install via fisher install mserajnik/fish-completions-pass-otp if you are using the Fisher plugin manager for fish.

@zvyn I have added your name to the copyright since I originally took the completions straight from this PR and only adjusted the descriptions slightly. My code is licensed as GPL-3.0-or-later, just like pass-otp itself (and thus also your fork of it). If this is not fine with you, simply let me know and I will remove your original completions (and your name) from my repository and rewrite them in a different way to distinguish them from your PR.
Edit: I have now added completions for all of pass-otp's subcommands too and have rewritten/expanded all but the top-level completion. Still, if you have any issues with your name appearing in my repository, just let me know.