completions: add fish shell support

[considerate]

This PR adds completions for the fish shell by wrapping the commands nix, nix-build and nix-shell.

Currently the nix package in nixpkgs only provides completions for the nix command so only completions on the nom command works. Whenever the package provides completions for nix-build and nix-shell the wrappers for nom-build and nom-shell will pick up those completions as well.

The completions do not try to filter based on the subcommand so nom search is completed even though it doesn't seem to be supported by the nom command.

[RossSmyth]

Looks good. Though I do not have completions for nix-build or nix-shell installed. But I do for the nix command.

[considerate]

@RossSmyth would you prefer me to drop the wrappers for nix-build and nix-shell since they're not installed at this moment?

[maralorn]

@considerate Thank you for your contribution and sorry for not responding. There is not much harm in merging this, I guess. Have you however tested that it works for the nom command? I am torn on the other two. Do you expect there to be a change in the provided nix completions? If not I think we should drop it.

Besides that for this to be useful I think we will need to add a line to the nix expression here and in nixpkgs after the next release, right?

[considerate]

@maralorn Thank you. Sorry, I didn't notice that that you replied here.

Have you however tested that it works for the nom command?

Yes, I have been using these completions locally for a couple of months now and I haven't come across any issues with them.

Do you expect there to be a change in the provided nix completions? If not I think we should drop it.

I don't have any insights as to whether there's anything planned. I'll go ahead and remove the redundant completions.