feat(comp): Add support for fish completion

[marckhouzam]

Part of #8109
Replaces #8111

What this PR does / why we need it:

Provides support for Fish shell completion through Cobra

Special notes for your reviewer:

This PR replaces #8111 because the contributor has not come back to his PR, which needs a rebase, since July 7th, 2020.
I've reached out three times but no luck.

Also, a new flag --no-descriptions was added just for the command helm completion fish, to allow users to disable descriptions if they don't like to have them. Descriptions are a feature of fish completion (and native zsh completion) that looks like this:

$ bin/helm s<TAB>
search  (search for a keyword in charts)  
show    (show information of a chart)  
status  (display the status of the named release)

[mattfarina]

lgtm.

Manually testing this out was fun. There are some things I like about fish that I forget about.

[marckhouzam]

Thanks @mattfarina.
I also liked fish after trying out. The use of colors is nice, the scripting language is more intuitive, but the feature that I enjoy the most was the grayed-out command-line that recalls my previous similar command. I have installed a plugin in zsh to mimic that last feature, I can't live without it anymore.

[thernstig]

@marckhouzam and @mattfarina - I just recently tested this, but assume it has not made it in to any release yet, and that it can be expected in Helm 3.4.0.

But I also noticed that this PR forgot to update the documentation, more specifically found here:
https://helm.sh/docs/helm/helm_completion/

Also note that there should be a documentation entry at this location: https://helm.sh/docs/helm/helm_completion_fish/

[marckhouzam]

Thank you @thernstig for trying this out. You are right, it will be part of Helm 3.4.
From what I know, the docs you mention are automatically generated and should therefore be updated once 3.4 has been released.

If you notice anything unexpected in your usage of Fish completion, I encourage you to continue reporting it, it will help others.